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."

app/controllers/devise/registrations_controller.rb

@@ -10,7 +10,7 @@ class Devise::RegistrationsController < DeviseController
 
   # POST /resource
   def create
-    self.resource = build_resource(sign_up_params)
+    build_resource(sign_up_params)
 
     if resource.save
       if resource.active_for_authentication?