wininet: Replace any existing cookie header.

dlls/wininet/http.c

@@ -3227,7 +3227,7 @@ static void HTTP_InsertCookies(LPWININETHTTPREQW lpwhr)
             InternetGetCookieW(lpszUrl, NULL, lpszCookies + cnt, &nCookieSize);
             strcatW(lpszCookies, szCrLf);
 
-            HTTP_HttpAddRequestHeadersW(lpwhr, lpszCookies, strlenW(lpszCookies), HTTP_ADDREQ_FLAG_ADD);
+            HTTP_HttpAddRequestHeadersW(lpwhr, lpszCookies, strlenW(lpszCookies), HTTP_ADDREQ_FLAG_REPLACE);
             HeapFree(GetProcessHeap(), 0, lpszCookies);
         }
     }

dlls/wininet/tests/http.c

@@ -1885,6 +1885,15 @@ static void test_cookie_header(int port)
     ok(!ret, "HttpQueryInfo succeeded\n");
     ok(error == ERROR_HTTP_HEADER_NOT_FOUND, "got %u expected ERROR_HTTP_HEADER_NOT_FOUND\n", error);
 
+    ret = HttpAddRequestHeaders(req, "Cookie: cookie=not biscuit\r\n", ~0u, HTTP_ADDREQ_FLAG_ADD);
+    ok(ret, "HttpAddRequestHeaders failed: %u\n", GetLastError());
+
+    buffer[0] = 0;
+    size = sizeof(buffer);
+    ret = HttpQueryInfo(req, HTTP_QUERY_COOKIE | HTTP_QUERY_FLAG_REQUEST_HEADERS, buffer, &size, NULL);
+    ok(ret, "HttpQueryInfo failed: %u\n", GetLastError());
+    ok(!strcmp(buffer, "cookie=not biscuit"), "got '%s' expected \'cookie=not biscuit\'\n", buffer);
+
     ret = HttpSendRequest(req, NULL, 0, NULL, 0);
     ok(ret, "HttpSendRequest failed: %u\n", GetLastError());