Fixed #16731 -- Made pattern lookups work properly with F() expressions

See https://code.djangoproject.com/ticket/16731#comment:11
Good catch. Actually there is no need to use raw string here (it should be like https://github.com/django/django/pull/3284/files#diff-54b46d05e1da568b3cc987c423e00c50R588). I removed it.