Checking for message fields in cache models when ProtoModel is called. Fixes issue 2.
Checking for message fields in cache models when ProtoModel is called. Fixes issue 2.

Reviewed in https://codereview.appspot.com/6931051/

jbergant/endpoints-proto-datastore,dhermes/endpoints-proto-datastore,maxandron/endpoints-proto-datastore,GoogleCloudPlatform/endpoints-proto-datastore,mnieper/endpoints-proto-datastore