Assigning nil to a polymorphic belongs_to should nullify its _type column

Assigning nil to a polymorphic belongs_to should nullify its _type column

Fixes failing test introduced at c141dfc838a5dca9f197814410fa5d44c143129c by @jeremy
The regression was introduced by 1678e959e973de32287b65c52ebc6cce87148951