Add tests for ActiveSupport::Deprecation.deprecate_methods
Modify ActiveSupport::Testing::Deprecation to allow a custom deprecator
Leverage ActiveSupport::Testing::Deprecation assert_deprecated
Update documentation for ActiveSupport::Deprecation.deprecate_methods

Use cases:
Using the default deprecator => "removed from Rails X.Y"
Passing a custom deprecator in the options hash => "removed from MyGem next-release"
Deprecating methods directly from custom deprecator => "removed from MyGem next-release"