Improve unions API
Improve unions API

Summary:
This patch allows users to set values to unions using accessor methods when the
union is empty.

Reviewed By: spalamarchuk

Differential Revision: D4926552

fbshipit-source-id: 31a7419d3bb7cada383e820ec352b0350ebe3766

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter