nghttpx: Remove Connection: close header field from CONNECT response

It appears that some Android client does not like Connection: close in
200 CONNECT response.  Browsers fine with this header field.  It is
better to remove it.  Squid does not emit it too.

src/shrpx_https_upstream.cc

@@ -676,7 +676,8 @@ int HttpsUpstream::on_downstream_header_complete(Downstream *downstream)
       // We add this header for HTTP/1.0 or HTTP/0.9 clients
       hdrs += "Connection: Keep-Alive\r\n";
     }
-  } else {
+  } else if(!downstream->get_upgraded() ||
+            downstream->get_request_method() != "CONNECT") {
     hdrs += "Connection: close\r\n";
   }