py3: use system strings for HTTP response header comparison

res.headers is holding system strings. Attempting to compare against
bytes always fails and we fail to print decoded CBOR responses on
Python 3.

After this change, various test-wireproto* tests are now properly
printing CBOR response objects.

Differential Revision: https://phab.mercurial-scm.org/D4835

mercurial/debugcommands.py

@@ -3324,7 +3324,8 b' def debugwireproto(ui, repo, path=None, '
                 getattr(e, 'read', lambda: None)()
                 continue
 
-            if res.headers.get('Content-Type') == 'application/mercurial-cbor':
+            ct = res.headers.get(r'Content-Type')
+            if ct == r'application/mercurial-cbor':
                 ui.write(_('cbor> %s\n') %
                          stringutil.pprint(cborutil.decodeall(body)[0],
                                            bprefix=True,