The `ActiveSupport::DeepMergeable` module allows a class to provide
`deep_merge` and `deep_merge!` methods simply by implementing a
`merge!(other, &block)` method.  Values will be deep merged only when
they are compatible, according to `deep_merge?`.  By default, that only
includes instances of the same class or its subclasses.  A class may
override `deep_merge?` to further restrict or expand the domain of deep
mergeable values.

This does introduce a small change in behavior.  Previously,
`Hash#deep_merge` would only deep merge `Hash` instances.  Now,
`deep_merge` will deep merge any `DeepMergeable` instances that are
compatible with each other.