url: fix a bytes vs str crash in processing proxy headers (issue6249)

I have no idea how to make a test for this, so if somebody knows, feel free to
add one or follow up on this.  The bug reporter reported that it worked for
them, so there may not be other hidden issues here.

Differential Revision: https://phab.mercurial-scm.org/D8485

mercurial/url.py

@@ -225,7 +225,7 b' def _generic_start_transaction(handler, '
 
 def _generic_proxytunnel(self):
     proxyheaders = {
-        x: self.headers[x]
+        pycompat.bytestr(x): pycompat.bytestr(self.headers[x])
         for x in self.headers
         if x.lower().startswith('proxy-')
     }