Don't prepend scheme and host to path if path starts with scheme.

This is just a sanity check in caes of path contains absoluteURI.

examples/shrpx_spdy_upstream.cc

@@ -169,7 +169,8 @@ void on_ctrl_recv_callback
       upstream->rst_stream(downstream, SPDYLAY_INTERNAL_ERROR);
       return;
     }
-    if(get_config()->spdy_proxy && scheme) {
+    if(get_config()->spdy_proxy && scheme &&
+       !util::istartsWith(path, scheme)) {
       std::string reqpath = scheme;
       reqpath += "://";
       reqpath += host;