Implemented ObjectProxy.SourceGeneration.
Implemented ObjectProxy.SourceGeneration.

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