Implemented ProxyObject.WriteAt.
Implemented ProxyObject.WriteAt.

GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,GoogleCloudPlatform/gcsfuse,jacobsa/timeutil,mbrukman/gcsfuse,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,kahing/gcsfuse,jacobsa/ratelimit