FastCGI: skip special bufs in buffered request body chain.

FastCGI: skip special bufs in buffered request body chain.

This prevents forming empty records out of such buffers.  Particularly it fixes
double end-of-stream records with chunked transfer encoding, or when HTTP/2 is
used and the END_STREAM flag has been sent without data.  In both cases there
is an empty buffer at the end of the request body chain with the "last_buf"
flag set.

The canonical libfcgi, as well as php implementation, tolerates such records,
while the HHVM parser is more strict and drops the connection (ticket #950).