XrdHTTP thread looping after partial chunk requests: critical bugfix. 

The nightly HC tests that include XrdHTTP degraded mid-December 2014 after upgrading to
Xrootd/XrdHTTP from EPEL-test in our stable testbed. Before it was running a head from October.

The symptom is thread looping after partial chunk requests under certain circumstances.
The consequence is the performance degrading to almost zero due to cpu consumption.

This pull request fixes this