remove AM delegating register_observer and register_interceptor to Mail and instead implement smarter versions allowing for string class names, also added proper Railtie support with tests.
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

[#6550 state:committed]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

This was conflicting with ActionView::Helpers::TextHelper#simple_format
from within email templates.

This reverts commit 1d482693.

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

Update ActionMailer documentation to not use deprecated template_root method as documentation, but rather raise_delivery_errors method

class inheritable attributes is used no more! all internal use of class inheritable has been changed to class_attribute. class inheritable attributes has been deprecated.
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

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

Set only helpers_path on inherited hook in action_controller/railtie.rb and use helper(:all) just after that

This change is needed, because we must take namespace into account and if
controller's/mailer's class is namespaced, engine's paths should be set
instead of application's ones.

The nice side effect of this is removing unneeded logic in
ActionController::Base.inherited - now the helpers_path should be set
correctly even for engine's controllers, so helper(:all) will always
include correct helpers.