serve: send response headers even if response has no body

The headers would usually be sent anyway because the app did a number of writes
of empty strings.

mercurial/hgweb/server.py

@@ -136,6 +136,8 b' class _httprequesthandler(BaseHTTPServer'
         self.length = None
         for chunk in self.server.application(env, self._start_response):
             self._write(chunk)
+        if not self.sent_headers:
+            self.send_headers()
 
     def send_headers(self):
         if not self.saved_status: