Fixes by John Reese and Jacques Frechet that make test_xmlrpc pass.

(Note that test_xmlrpc isn't touched by the fixes!)  There were two
separate issues; (a) BaseHTTPServer was using a TextIOWrapper which
was swallowing some of the POST body; (b) the getheaders() API was
changed but (due to integration of 2.6 code) the code wasn't modified.