Fix map copy in THeader::releaseWriteHeaders()
Fix map copy in THeader::releaseWriteHeaders()

Summary: See note in code comment. In short, the old code was prevented from moving because of its use of the conditional operator (`?:`).

Reviewed By: iahs

Differential Revision: D37666103

fbshipit-source-id: 00420bf17de900c2146d2aa31909ac29b7c8c4fe

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