mod_proxy_http: follow up to r1836588: nonblocking read for 100-continue body.

Set nonblocking read (req->flushall) when handling 100-continue since no body
is expected to be there already.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1837040 13f79535-47bb-0310-9956-ffa450edef68