ActiveRecord and Mongoid adapters now acceps nil for role_name on remove

[Genkilabs]

This is my solution for #426

Use case: I want to remove all the roles a user has for some resource, without listing them individually.

Basically this allows for syntax current_user.remove_role nil, some_resource.

[coveralls]

Coverage increased (+0.007%) to 93.096% when pulling 1dd66ab on Genkilabs:master into 154748c on RolifyCommunity:master.