http_proxy: do not count proxy headers in the header bytecount

... as that counter is subsequently used to detect if nothing was
returned from the peer. This made curl return CURLE_OK when it should
have returned CURLE_GOT_NOTHING.

Fixes #5992
Reported-by: Tom van der Woerdt
Closes #5994

lib/http_proxy.c

@@ -425,7 +425,6 @@ static CURLcode CONNECT(struct connectdata *conn,
         }
 
         data->info.header_size += (long)perline;
-        data->req.headerbytecount += (long)perline;
 
         /* Newlines are CRLF, so the CR is ignored as the line isn't
            really terminated until the LF comes. Treat a following CR