support Relation object saving

[yuki-takeichi]

This PR fixes a compatibility issue for saving Relation object.

Request:

curl -X POST \
  -H 'X-Parse-Application-Id: XXX' \
  -H 'X-Parse-Master-Key:  YYY' \
  -H 'Content-Type: application/json' \
  -d '{"aRelation":{"__type":"Relation","className":"Foo"}}' \
https://api.parse.com/1/classes/Foo

Response from api.parse.com:

{"createdAt":"2016-11-19T11:53:04.087Z","objectId":"blEqHPUBuk"}

(Parse Class Foo and its field aRelation are created implicitly.)

Response from parse-server:

{"code":111,"error":"This is not a valid Relation"}