wsgiheaders: make sure __repr__() returns a string

When printing `req.headers` on the server side to debug, it complained that '%b'
needed to take a string, not bytes.  Changing '%s' to '%r' caused it to complain
that __repr__ didn't return a string.

mercurial/hgweb/wsgiheaders.py

@@ -127,7 +127,7 b' class Headers(object):'
         return self._headers[:]
 
     def __repr__(self):
-        return "%s(%r)" % (self.__class__.__name__, self._headers)
+        return r"%s(%r)" % (self.__class__.__name__, self._headers)
 
     def __str__(self):
         """str() returns the formatted headers, complete with end line,