fix Content-Encoded: chunked for some edge case

These seems to fix the problem I have with Content-Encoding: chunked in my setup, problem not existing in others setup... Maybe a php.ini settings about content buffering.

Anyway, without this fix, I can't get Chunked encoding working right, the output is chunk-encoded two times : the inner buffer is (partially) encoded, then the outer buffer is correctly encoded, but leaving the inner one with unfinished chunks...