hgweb: insist http_status value is a sysstr

My previous change was extremely confusing to figure out, because I
thought I was looking at a client-side problem. For the low cost of an
isinstance assert, we can make those errors not happen in the future.

Differential Revision: https://phab.mercurial-scm.org/D3727

mercurial/hgweb/server.py

@@ -215,6 +215,7 b' class _httprequesthandler(httpservermod.'
         self.sent_headers = True
 
     def _start_response(self, http_status, headers, exc_info=None):
+        assert isinstance(http_status, str)
         code, msg = http_status.split(None, 1)
         code = int(code)
         self.saved_status = http_status