Curl_read: check for activated HTTP/1 pipelining, not only requested

... as when pipelining is used, we read things into a unified buffer and
we don't do that with HTTP/2. This could then easily make programs that
set CURLMOPT_PIPELINING = CURLPIPE_HTTP1|CURLPIPE_MULTIPLEX to get data
intermixed or plain broken between HTTP/2 streams.

Reported-by: Anders Bakken