Fix redundant assignment of self.resource in Devise::RegistrationsController

Devise::RegistrationsController#create set self.resource to the return value of build_resource--which is nil, because build_resource sets self.resource to an actual resource object. This caused attempting to save the resource (two lines down) to fail with "undefined method `save' on nil:NilClass."
Merge pull request #2413 from jbhannah/master

Fix redundant assignment of self.resource in Devise::RegistrationsController