wireprotoserver: headers are bytes for us internally, use bytes

This re-fixes test-pull-http.t on Python 3. Probably many others as well.

Differential Revision: https://phab.mercurial-scm.org/D3249

mercurial/wireprotoserver.py

@@ -103,7 +103,7 b' class httpv1protocolhandler(object):'
 
     def getprotocaps(self):
         if self._protocaps is None:
-            value = decodevaluefromheaders(self._req, r'X-HgProto')
+            value = decodevaluefromheaders(self._req, b'X-HgProto')
             self._protocaps = set(value.split(' '))
         return self._protocaps