request: coerce content-type to native str

Again, required by WSGI.

Differential Revision: https://phab.mercurial-scm.org/D969

mercurial/hgweb/request.py

@@ -19,6 +19,7 b' from .common import ('
 )
 
 from .. import (
+    pycompat,
     util,
 )
 
@@ -89,6 +90,8 b' class wsgirequest(object):'
             pass
 
     def respond(self, status, type, filename=None, body=None):
+        if not isinstance(type, str):
+            type = pycompat.sysstr(type)
         if self._start_response is not None:
             self.headers.append(('Content-Type', type))
             if filename: