twitter: don't count filter stream reply as valid pongs
Twitter streams send newlines to indicate that they are alive. The
twitter_http_stream() function processes those and sets the ponged
flag so that the whole connection doesn't timeout.
That function is used to handle both user stream and filter stream.
If the user stream is dead (not sending whitespace) but the filter
stream isn't, the latter keeps the connection alive while the main
twitter channel is completely dead.
This commit only sets the ponged flag for the user stream. This has
the side effect of not detecting if the filter stream dies - but that
didn't work before, anyway. In the future the whole stream connection
management should be revamped - for example stream disconnections
shouldn't take the whole account down, especially not filter streams.
