*   `Module#delegate_missing_to` now raises `DelegationError` if target is nil,
    similar to `Module#delegate`.

    *Anton Khamets*

*   Update `String#camelize` to provide feedback when wrong option is passed

    `String#camelize` was returning nil without any feedback when an
    invalid option was passed as a parameter.

    Previously:

        'one_two'.camelize(true)
        => nil

    Now:

        'one_two'.camelize(true)
        => ArgumentError: Invalid option, use either :upper or :lower.

    *Ricardo Díaz*

*   Fix modulo operations involving durations

    Rails 5.1 introduced `ActiveSupport::Duration::Scalar` as a wrapper
    around numeric values as a way of ensuring a duration was the outcome of
    an expression. However, the implementation was missing support for modulo
    operations. This support has now been added and should result in a duration
    being returned from expressions involving modulo operations.

    Prior to Rails 5.1:

        5.minutes % 2.minutes
        => 60

    Now:

        5.minutes % 2.minutes
        => 1 minute

    Fixes #29603 and #29743.

    *Sayan Chakraborty*, *Andrew White*

*   Fix division where a duration is the denominator

    PR #29163 introduced a change in behavior when a duration was the denominator
    in a calculation - this was incorrect as dividing by a duration should always
    return a `Numeric`. The behavior of previous versions of Rails has been restored.

    Fixes #29592.

    *Andrew White*

*   Add purpose and expiry support to `ActiveSupport::MessageVerifier` &
   `ActiveSupport::MessageEncryptor`.

    For instance, to ensure a message is only usable for one intended purpose:

        token = @verifier.generate("x", purpose: :shipping)

        @verifier.verified(token, purpose: :shipping) # => "x"
        @verifier.verified(token)                     # => nil

    Or make it expire after a set time:

        @verifier.generate("x", expires_in: 1.month)
        @verifier.generate("y", expires_at: Time.now.end_of_year)

    Showcased with `ActiveSupport::MessageVerifier`, but works the same for
    `ActiveSupport::MessageEncryptor`'s `encrypt_and_sign` and `decrypt_and_verify`.

    Pull requests: #29599, #29854

    *Assain Jaleel*

*   Make the order of `Hash#reverse_merge!` consistent with `HashWithIndifferentAccess`.

    *Erol Fornoles*

*   Add `freeze_time` helper which freezes time to `Time.now` in tests.

    *Prathamesh Sonpatki*

*   Default `ActiveSupport::MessageEncryptor` to use AES 256 GCM encryption.

    On for new Rails 5.2 apps. Upgrading apps can find the config as a new
    framework default.

    *Assain Jaleel*

*   Cache: `write_multi`

        Rails.cache.write_multi foo: 'bar', baz: 'qux'

    Plus faster fetch_multi with stores that implement `write_multi_entries`.
    Keys that aren't found may be written to the cache store in one shot
    instead of separate writes.

    The default implementation simply calls `write_entry` for each entry.
    Stores may override if they're capable of one-shot bulk writes, like
    Redis `MSET`.

    *Jeremy Daer*

*   Add default option to module and class attribute accessors.

        mattr_accessor :settings, default: {}

    Works for `mattr_reader`, `mattr_writer`, `cattr_accessor`, `cattr_reader`,
    and `cattr_writer` as well.

    *Genadi Samokovarov*

*   Add `Date#prev_occurring` and `Date#next_occurring` to return specified next/previous occurring day of week.

    *Shota Iguchi*

*   Add default option to `class_attribute`.

    Before:

        class_attribute :settings
        self.settings = {}

    Now:

        class_attribute :settings, default: {}

    *DHH*

*   `#singularize` and `#pluralize` now respect uncountables for the specified locale.

    *Eilis Hamilton*

*   Add `ActiveSupport::CurrentAttributes` to provide a thread-isolated attributes singleton.
    Primary use case is keeping all the per-request attributes easily available to the whole system.

    *DHH*

*   Fix implicit coercion calculations with scalars and durations

    Previously, calculations where the scalar is first would be converted to a duration
    of seconds, but this causes issues with dates being converted to times, e.g:

        Time.zone = "Beijing"           # => Asia/Shanghai
        date = Date.civil(2017, 5, 20)  # => Mon, 20 May 2017
        2 * 1.day                       # => 172800 seconds
        date + 2 * 1.day                # => Mon, 22 May 2017 00:00:00 CST +08:00

    Now, the `ActiveSupport::Duration::Scalar` calculation methods will try to maintain
    the part structure of the duration where possible, e.g:

        Time.zone = "Beijing"           # => Asia/Shanghai
        date = Date.civil(2017, 5, 20)  # => Mon, 20 May 2017
        2 * 1.day                       # => 2 days
        date + 2 * 1.day                # => Mon, 22 May 2017

    Fixes #29160, #28970.

    *Andrew White*

*   Add support for versioned cache entries. This enables the cache stores to recycle cache keys, greatly saving
    on storage in cases with frequent churn. Works together with the separation of `#cache_key` and `#cache_version`
    in Active Record and its use in Action Pack's fragment caching.

    *DHH*

*   Pass gem name and deprecation horizon to deprecation notifications.

    *Willem van Bergen*

*   Add support for `:offset` and `:zone` to `ActiveSupport::TimeWithZone#change`

    *Andrew White*

*   Add support for `:offset` to `Time#change`

    Fixes #28723.

    *Andrew White*

*   Add `fetch_values` for `HashWithIndifferentAccess`

    The method was originally added to `Hash` in Ruby 2.3.0.

    *Josh Pencheon*


Please check [5-1-stable](https://github.com/rails/rails/blob/5-1-stable/activesupport/CHANGELOG.md) for previous changes.