1.8.0: Set missing header_namespace

Summary: When we don't explicitly set header_namespace, we can get differing behavior between Android and iOS. Even when it is not explicitly necessary, we are going to try to enforce this properly.

Reviewed By: scottrice

Differential Revision: D7141155

fbshipit-source-id: 74d1488c3909f89ad9d6d0ee0de335b789dc2ccd