IPRO: Fix a race in processing writes() from NgxBaseFetch

For FetchInPlaceResource, NgxBaseFetch would send two bytes down its
pipe, one upon HeaderComplete() and one upon HandleDone(). We need
only one to resume processing on the nginx side.

There is a race between ps_connection_read_handler() and processing
of the byte send by NgxBaseFetch::HandleDone().
ps_connection_read_handler() clears the pipe when the request is
finalized, and also drains it on each event - so two writes could be
processed as one when lucky, masking the problem).

One concrete problem this solved for me was that SPDY + IPRO +
proxy_pass would segfault, hang, and/or pass on 5xx/404 responses
from IPRO lookup fetches to the browser, next to alerts about
r->count being zero in nginx's error.log

Might fix https://github.com/pagespeed/ngx_pagespeed/issues/788
Fixes https://github.com/pagespeed/ngx_pagespeed/issues/792
Merge pull request #794 from pagespeed/oschaaf-ipro-race

IPRO: Fix a race in processing writes() from NgxBaseFetch