Remove hop headers

Requested by @LucasRoesler - removes headers detailed in HTTP
spec which are not supposed to be forwarded by proxies or
gateways.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/handlers/forwarding_proxy_test.go

@@ -338,3 +338,22 @@ func Test_buildUpstreamRequest_WithPathAndQuery(t *testing.T) {
 	}
 
 }
+
+func Test_deleteHeaders(t *testing.T) {
+	h := http.Header{}
+	target := "X-Dont-Forward"
+	h.Add(target, "value1")
+	h.Add("X-Keep-This", "value2")
+
+	deleteHeaders(&h, &[]string{target})
+
+	if h.Get(target) == "value1" {
+		t.Errorf("want %s to be removed from headers", target)
+		t.Fail()
+	}
+
+	if h.Get("X-Keep-This") != "value2" {
+		t.Errorf("want %s to remain in headers", "X-Keep-This")
+		t.Fail()
+	}
+}