c-hyper: avoid duplicated Proxy-Connection headers

lib/c-hyper.c

@@ -800,6 +800,7 @@ CURLcode Curl_http(struct Curl_easy *data, bool *done)
 
 #ifndef CURL_DISABLE_PROXY
   if(conn->bits.httpproxy && !conn->bits.tunnel_proxy &&
+     !Curl_checkheaders(data, "Proxy-Connection") &&
      !Curl_checkProxyheaders(data, conn, "Proxy-Connection")) {
     if(Curl_hyper_header(data, headers, "Proxy-Connection: Keep-Alive"))
       goto error;