add content negotiation feature, add context.ReadYAML and fix https://github.com/kataras/neffos/issues/1#issuecomment-515698536

Former-commit-id: 9753e3e45c7c24788b97814d3ecfb4b03f5ff414

_examples/http_request/read-yaml/main_test.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/kataras/iris/httptest"
+)
+
+func TestReadYAML(t *testing.T) {
+	app := newApp()
+	e := httptest.New(t, app)
+
+	expectedResponse := `Received: main.product{Invoice:34843, Tax:251.42, Total:4443.52, Comments:"Late afternoon is best. Backup contact is Nancy Billsmer @ 338-4338."}`
+	send := `invoice: 34843
+tax  : 251.42
+total: 4443.52
+comments: >
+    Late afternoon is best.
+    Backup contact is Nancy
+    Billsmer @ 338-4338.`
+
+	e.POST("/").WithHeader("Content-Type", "application/x-yaml").WithBytes([]byte(send)).Expect().
+		Status(httptest.StatusOK).Body().Equal(expectedResponse)
+}