closes #5970

The latter doesn't render as code in HTML output.

Regex used in Rubymine to locate the latter form:

    (\+)(:*\w+:(?::|\w)+)(\+)

Prior to this commit, there isn't a good way of adding things like
default inline attachments to an email.  This Stack Overflow thread
shows people using hooks like the 'default' method in ActionMailer::Base
to call a Proc for message configuration:

http://stackoverflow.com/questions/5113121/rails-use-same-attachment-for-all-emails-using-layout

This has the unintended side effect of setting a message header, so it's not a good solution.

This pull request adds support for message modifications by including AbstractController:Callbacks
in ActionMailer::Base. It includes tests and documentation for the functionality
provided by including this module.

Proc#bind is not useful when called immediately and previous check for #call method is not correct

- instance_variables return symbols in 1.9
- there is instance_variable_defined? method

The :actionmailer i18n scope part is not used and according to https://github.com/rails/rails/pull/2542 the implementation is correct.

The name for the plain text template in Rails3 appears to be method.text.erb, not, method.text.plain.erb.  Updated the doc to reflect this change.  Also fixed a reference to an example that was incorrect.

This made the isolated test failed on CI server.
Signed-off-by: NXavier Noria <fxn@hashref.com>

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>