Fix ThriftSerializationHelper for php style sets
Fix ThriftSerializationHelper for php style sets

Summary: php style sets have the items as the keys and the values as true. Prior to this, we were mistakenly sending the literal true over the wire for every item instead of the items themselves.

Reviewed By: yfeldblum

Differential Revision: D7168131

fbshipit-source-id: 065a811fc71079644f1336d21eefd16ecf4d48c7

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift