Before this commit, returning `false` in an ActiveModel `before_` callback
such as `before_create` would halt the callback chain.
After this commit, the behavior is deprecated: will still work until
the next release of Rails but will also display a deprecation warning.
The preferred way to halt a callback chain is to explicitly `throw(:abort)`.
This stems from https://github.com/rails/rails/pull/17227#discussion_r21641358
It's simply a clarification of the current behavior by which if an
`after_` or `around_` ActiveModel callback returns +false+, then the callback
chain **is not halted**.
The callback chain in ActiveModel is only halted when a `before_`
callback returns `false`.
