Add notifications to ActionDispatch::ShowExceptions, this can be used as hooks for plugins like ExceptionNotifier.

Updating to Mail 1.5.0, including default values for all Message#field_name methods, can access field objects by calling Message#[:field_name]

Reapply "Remove optional join_dependency argument as Relation always supplies it" - Now without syntax errors

[#2982 state:committed]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

This reverts commit 8e57deed.

(Thor actually accepts both, the patch was applied to improve readability)
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Remove unused code from association.rb now that Relation takes care of checking the referenced tables

Remove deprecated ActiveModel tests (%d and %s is no longer supported in error messages a couple months already)

Signed-off-by: NJosé Valim <jose.valim@gmail.com>