AR will_change! and :mount_on

[indrekj]

mount_uploader :avatar, AvatarUploader, :mount_on => :avatar_name

When I upload a new avatar then I get an error:
NoMethodError (undefined method `avatar_will_change!' for #<>.

This is because carrierwave/orm/activerecord.rb doesn't respect mount_on parameter.

[trevorturk]

Ah, yes, I think this line needs to respect the mount_on option:

https://github.com/jnicklas/carrierwave/blob/126832280f34be8cc90ba296bbff4e662368ed3f/lib/carrierwave/orm/activerecord.rb#L33

Any chance you could work up a pull request (with a new test) ?

[indrekj]

#276
Here. I hope it is good enough. I'm not very familiar with this gem source code.