Fix missing tracing backend in documentation

docs/configuration/tracing.md

@@ -4,7 +4,7 @@ Tracing system allows developers to visualize call flows in there infrastructure
 
 We use [OpenTracing](http://opentracing.io). It is an open standard designed for distributed tracing.
 
-Træfik supports two backends: Jaeger and Zipkin.
+Træfik supports three tracing backends: Jaeger, Zipkin and DataDog.
 
 ## Jaeger
 

middlewares/tracing/entrypoint_test.go

@@ -17,6 +17,7 @@ func TestEntryPointMiddlewareServeHTTP(t *testing.T) {
 		"http.url":    "http://www.test.com",
 		"http.host":   "www.test.com",
 	}
+
 	testCases := []struct {
 		desc         string
 		entryPoint   string