Add integration test to Airbrake hook

Add a test for the Airbrake hook to:

a) document how the hook is intended to work

b) test that an XML payload is received with the expected message

hooks/airbrake/airbrake_test.go

@@ -0,0 +1,57 @@
+package logrus_airbrake
+
+import (
+	"encoding/xml"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/tobi/airbrake-go"
+)
+
+type notice struct {
+	Error struct {
+		Message string `xml:"message"`
+	} `xml:"error"`
+}
+
+func TestNoticeReceived(t *testing.T) {
+	msg := make(chan string, 1)
+	expectedMsg := "foo"
+
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		var notice notice
+		if err := xml.NewDecoder(r.Body).Decode(&notice); err != nil {
+			t.Error(err)
+		}
+		r.Body.Close()
+
+		msg <- notice.Error.Message
+	}))
+	defer ts.Close()
+
+	hook := &AirbrakeHook{}
+
+	airbrake.Environment = "production"
+	airbrake.Endpoint = ts.URL
+	airbrake.ApiKey = "foo"
+
+	log := logrus.New()
+	log.Hooks.Add(hook)
+
+	log.WithFields(logrus.Fields{
+		"error": errors.New(expectedMsg),
+	}).Error("Airbrake will not see this string")
+
+	select {
+	case received := <-msg:
+		if received != expectedMsg {
+			t.Errorf("Unexpected message received: %s", received)
+		}
+	case <-time.After(time.Second):
+		t.Error("Timed out; no notice received by Airbrake API")
+	}
+}