Support Rails's `attribute` in Cancancan's `permitted_attributes` about cancancan

[kalsan]

I have created pull request #839 to address this. This pull request fixes the core issue where cancancan misses attribute :foo in general.

Rail's method has_secure_password is still missed, because for some reason, Rails does not declare an explicit attribute for password and password_confirmation. I don't think there is a simple way to detect those (and hardcoding a search for those two methods is likely a terrible idea), so I'd suggest mentionning in the documentation that for this particular case, cancancan can be brought to "see" the extra attributes by typing:

class User < ApplicationRecord
  has_secure_password
  attribute :password # perhaps, add additional arguments such as the type (string)
  attribute :password_confirmation # as above
end

Looking forward to hear what you think :-)