- Return 0 on doh_encode error. This makes the values returned on error
  by doh_encode() consistent with the post-invocation test.

Prior to this change doh_encode, which returns a size_t of bytes
written on success, erroneously returned DOHcodes on error
(eg DOH_TOO_SMALL_BUFFER (4), ie 4 bytes written, which was wrong).

Fixes #40
Closes #41