This bug was due to an interaction between two optimizations. If we
attempt to flush the buffer before any bytes are used we refused.
However, in enc_ensure we were not checking whether the buffer was
actually flushed so we would allocate a new buffer for the request.
The easiest way to encounter this issue was by encoding a raw binary
longer than 2041 bytes (i.e., `jiffy:encode(<<"stuff...">>).`).