Allow objects to specify their devise scope.

Introspecting the scope of an object can make it difficult to use
wrapper patterns. See issue plataformatec/devise#3307 for an example.

Allow objects to specify their scope explicitly by implementing
`devise_scope`.

lib/devise/mapping.rb

@@ -31,6 +31,7 @@ module Devise
     # Receives an object and find a scope for it. If a scope cannot be found,
     # raises an error. If a symbol is given, it's considered to be the scope.
     def self.find_scope!(obj)
+      obj = obj.devise_scope if obj.respond_to?(:devise_scope)
       case obj
       when String, Symbol
         return obj.to_sym

test/mapping_test.rb

@@ -71,6 +71,12 @@ class MappingTest < ActiveSupport::TestCase
     assert_equal :user, Devise::Mapping.find_scope!(Class.new(User).new)
   end
 
+  test 'find scope uses devise_scope' do
+    user = User.new
+    def user.devise_scope; :special_scope; end
+    assert_equal :special_scope, Devise::Mapping.find_scope!(user)
+  end
+
   test 'find scope raises an error if cannot be found' do
     assert_raise RuntimeError do
       Devise::Mapping.find_scope!(String)