Implemented ProxyObject.Truncate.
Implemented ProxyObject.Truncate.

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