Move config_accessor :asset_host from ActionController::Base to AbstractController which fixes issues with asset_host in ActionMailer

Including:
  - Moved mailer objects in separate directory
  - Added two tests for asset_host configuration option

Signed-off-by: NXavier Noria <fxn@hashref.com>

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

Performance optimizations to handle cases of instrumentors that are not listened to. Also, fix a possible concurrency issue.

options could be of any kind of Hash (Hash, HashWithIndifferentAccess or OrderedHash) this way we keep the properties of the options passed as an argument

camelize and underscore are sort of inverse of each other, but not in a mathematical sense [#5174 state:resolved]

Active Record Validations and Callbacks guide: Fixed typos and rephrased some paragraphs for clarity

[#5175]

[#5153 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>