Test Ensure unset called before set
Test Ensure unset called before set

Summary: This is a unittest for an issue I fixed previously (but had to land ASAP).

Reviewed By: yfeldblum

Differential Revision: D9140622

fbshipit-source-id: 09dcb4206d2a09a57c2f2239a1aa217b124e88fd

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly