raise if anchor is passed to scope

The `anchor` parameter [is overridden](https://github.com/rails/rails/blob/b4b4a611d0eb9aa1c640c5f521c6a43bf2a65bab/actionpack/lib/action_dispatch/routing/mapper.rb#L1528) unless it
is directly passed to `match`, so setting it in a scope must be a
mistake.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -788,6 +788,10 @@ def scope(*args)
                                        except: options.delete(:except) }
           end
 
+          if options.key? :anchor
+            raise ArgumentError, 'anchor is ignored unless passed to `match`'
+          end
+
           @scope.options.each do |option|
             if option == :blocks
               value = block

actionpack/test/dispatch/mapper_test.rb

@@ -44,6 +44,15 @@ def test_initialize
         Mapper.new FakeSet.new
       end
 
+      def test_scope_raises_on_anchor
+        fakeset = FakeSet.new
+        mapper = Mapper.new fakeset
+        assert_raises(ArgumentError) do
+          mapper.scope(anchor: false) do
+          end
+        end
+      end
+
       def test_blows_up_without_via
         fakeset = FakeSet.new
         mapper = Mapper.new fakeset