Issue #159: Panic on invalid response

The metered roundtripper is not checking whether the
response is valid before registering the status code.
This leads to a panic on that handler. This patch
fixes this behavior.

proxy/http.go

@@ -25,7 +25,9 @@ type meteredRoundTripper struct {
 func (m *meteredRoundTripper) RoundTrip(r *http.Request) (*http.Response, error) {
 	start := time.Now()
 	resp, err := m.tr.RoundTrip(r)
-	metrics.DefaultRegistry.GetTimer(name(resp.StatusCode)).UpdateSince(start)
+	if resp != nil {
+		metrics.DefaultRegistry.GetTimer(name(resp.StatusCode)).UpdateSince(start)
+	}
 	return resp, err
 }