hgweb: put response headers back into str for Python 3

This fixes a lot of hanging tests on Python 3, because
"Content-Length" was getting sent as "b'Content-Length'" (yes, really)
and then clients would expect a close-is-end body instead of counting
off a certain number of bytes.

Differential Revision: https://phab.mercurial-scm.org/D3248

mercurial/hgweb/request.py

@@ -518,8 +518,10 b' class wsgiresponse(object):'
                 if not chunk:
                     break
 
+        strheaders = [(pycompat.strurl(k), pycompat.strurl(v)) for
+                      k, v in self.headers.items()]
         write = self._startresponse(pycompat.sysstr(self.status),
-                                    self.headers.items())
+                                    strheaders)
 
         if self._bodybytes:
             yield self._bodybytes