List available representations in 406 body (#437)

Becasue RFC7231 says we should.

jsr311.go

@@ -147,7 +147,14 @@ func (r RouterJSR311) detectRoute(routes []Route, httpRequest *http.Request) (*R
 		if trace {
 			traceLogger.Printf("no Route found (from %d) that matches HTTP Accept: %s\n", len(previous), accept)
 		}
-		return nil, NewError(http.StatusNotAcceptable, "406: Not Acceptable")
+		available := []string{}
+		for _, candidate := range previous {
+			available = append(available, candidate.Produces...)
+		}
+		return nil, NewError(
+			http.StatusNotAcceptable,
+			fmt.Sprintf("406: Not Acceptable\n\nAvailable representations: %s", strings.Join(available, ", ")),
+		)
 	}
 	// return r.bestMatchByMedia(outputMediaOk, contentType, accept), nil
 	return candidates[0], nil

web_service_test.go

@@ -1,6 +1,7 @@
 package restful
 
 import (
+	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -104,6 +105,25 @@ func TestMethodNotAllowed_Issue435(t *testing.T) {
 	}
 }
 
+func TestNotAcceptable_Issue434(t *testing.T) {
+	tearDown()
+	Add(newGetPlainTextOrJsonService())
+	httpRequest, _ := http.NewRequest("GET", "http://here.com/get", nil)
+	httpRequest.Header.Set("Accept", "application/toml")
+	httpWriter := httptest.NewRecorder()
+	DefaultContainer.dispatch(httpWriter, httpRequest)
+	if 406 != httpWriter.Code {
+		t.Error("406 expected not acceptable", httpWriter.Code)
+	}
+	expected := `406: Not Acceptable
+
+Available representations: text/plain, application/json`
+	body, _ := ioutil.ReadAll(httpWriter.Body)
+	if expected != string(body) {
+		t.Errorf("Expected body:\n%s\ngot:\n%s\n", expected, string(body))
+	}
+}
+
 func TestSelectedRoutePath_Issue100(t *testing.T) {
 	tearDown()
 	Add(newSelectedRouteTestingService())