Return parameters enumerator from transform_keys/!

Previously calling `ActionController::Parameters#transform_keys/!`
without passing a block would return an enumerator for the underlying
hash, which was inconsistent with the behaviour when a block was passed:

    ActionController::Parameters.new(foo: "bar").transform_keys { |k| k }
    => <ActionController::Parameters {"foo"=>"bar"} permitted: false>
    ActionController::Parameters.new(foo: "bar").transform_keys.each { |k| k }
    => {"foo"=>"bar"}

An enumerator for the parameters is now returned instead, ensuring that
evaluating it produces another parameters object instead of a hash:

    ActionController::Parameters.new(foo: "bar").transform_keys.each { |k| k }
    => <ActionController::Parameters {"foo"=>"bar"} permitted: false>

0 files changed, 0 insertions, 0 deletions