Curl_proxyCONNECT: clear 'rewindaftersend' on success

After having done a POST over a CONNECT request, the 'rewindaftersend'
boolean could be holding the previous value which could lead to badness.

This should be tested for in a new test case!

Bug: https://groups.google.com/d/msg/msysgit/B31LNftR4BI/KhRTz0iuGmUJ