fix tests to work with go 1.8

internal/testhelper/testhelper.go

@@ -20,6 +20,11 @@ func AssertResponseBody(t *testing.T, response *httptest.ResponseRecorder, expec
 	}
 }
 
+func AssertResponseBodyRegexp(t *testing.T, response *httptest.ResponseRecorder, expectedBody *regexp.Regexp) {
+	if !expectedBody.MatchString(response.Body.String()) {
+		t.Fatalf("for HTTP request expected to receive body matching %q, got %q instead", expectedBody.String(), response.Body.String())
+	}
+}
 func AssertResponseHeader(t *testing.T, response *httptest.ResponseRecorder, header string, expectedValue string) {
 	if response.Header().Get(header) != expectedValue {
 		t.Fatalf("for HTTP request expected to receive the header %q with %q, got %q", header, expectedValue, response.Header().Get(header))

proxy_test.go

@@ -67,7 +67,7 @@ func TestProxyError(t *testing.T) {
 	w := httptest.NewRecorder()
 	newProxy("http://localhost:655575/", nil).ServeHTTP(w, httpRequest)
 	testhelper.AssertResponseCode(t, w, 502)
-	testhelper.AssertResponseBody(t, w, "dial tcp: invalid port 655575")
+	testhelper.AssertResponseBodyRegexp(t, w, regexp.MustCompile("dial tcp:.*invalid port.*"))
 }
 
 func TestProxyReadTimeout(t *testing.T) {