Accept a collection in fresh_when and stale?

The methods `fresh_when` and `stale?` from ActionController::ConditionalGet
accept a single record as a short form for a hash. For instance

```ruby
  def show
    @article = Article.find(params[:id])
    fresh_when(@article)
  end
```

is just a short form for:

```ruby
  def show
    @article = Article.find(params[:id])
    fresh_when(etag: @article, last_modified: @article.created_at)
  end
```

This commit extends `fresh_when` and `stale?` to also accept a collection
of records, so that a short form similar to the one above can be used in
an `index` action. After this commit, the following code:

```ruby
def index
  @article = Article.all
  fresh_when(etag: @articles, last_modified: @articles.maximum(:created_at))
end
```

can be simply written as:

```ruby
def index
  @article = Article.all
  fresh_when(@articles)
end
```

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