timecond: do not add if-modified-since without timecondition

The RTSP code path didn't skip adding the if-modified-since for certain
RTSP code paths, even if CURLOPT_TIMECONDITION was set to
CURL_TIMECOND_NONE.

Also, an unknown non-zero CURLOPT_TIMECONDITION value no longer equals
CURL_TIMECOND_IFMODSINCE.

Bug: http://stackoverflow.com/questions/33903982/curl-timecond-none-doesnt-work-how-to-remove-if-modified-since-header