http: make providing Proxy-Connection header not cause duplicated headers

Fixes test 1180

Bug: https://curl.se/mail/lib-2021-01/0095.html
Reported-by: Fabian Keil
Closes #6472

diff --git a/lib/http.c b/lib/http.c
index 993dc61af2c398b9fda0d0e84d836c2737137015..520ac1070bd8d24dc1b006fd2c009f0cb266bd5c 100644 (file)
--- a/lib/http.c
+++ b/lib/http.c
@@ -3123,6 +3123,7 @@ CURLcode Curl_http(struct Curl_easy *data, bool *done)
 #ifndef CURL_DISABLE_PROXY
                   (conn->bits.httpproxy &&
                    !conn->bits.tunnel_proxy &&
+                   !Curl_checkheaders(data, "Proxy-Connection") &&
                    !Curl_checkProxyheaders(data, conn, "Proxy-Connection"))?
                   "Proxy-Connection: Keep-Alive\r\n":"",
 #else