Fixed Upgrade header clearing with proxy_cache.

This was missed in proxy HTTP/1.1 support commit (r4127).

src/http/modules/ngx_http_proxy_module.c

diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index d8f7147..5476f39 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -543,6 +543,7 @@
     { ngx_string("Connection"), ngx_string("close") },
     { ngx_string("Keep-Alive"), ngx_string("") },
     { ngx_string("Expect"), ngx_string("") },
+    { ngx_string("Upgrade"), ngx_string("") },
     { ngx_string("If-Modified-Since"), ngx_string("") },
     { ngx_string("If-Unmodified-Since"), ngx_string("") },
     { ngx_string("If-None-Match"), ngx_string("") },