Avoid creating multiple regexp objects

lib/simple_form/form_builder.rb

@@ -676,7 +676,7 @@ def attempt_mapping(mapping, at)
       begin
         at.const_get(mapping)
       rescue NameError => e
-        raise if e.message !~ /#{mapping}$/
+        raise unless e.message.end_with?(mapping)
       end
     end