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.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"github.com/kataras/iris"
+)
+
+func newApp() *iris.Application {
+	app := iris.New()
+	app.Post("/", handler)
+
+	return app
+}
+
+// simple yaml stuff, read more at https://yaml.org/start.html
+type product struct {
+	Invoice  int     `yaml:"invoice"`
+	Tax      float32 `yaml:"tax"`
+	Total    float32 `yaml:"total"`
+	Comments string  `yaml:"comments"`
+}
+
+func handler(ctx iris.Context) {
+	var p product
+	if err := ctx.ReadYAML(&p); err != nil {
+		ctx.StatusCode(iris.StatusBadRequest)
+		ctx.WriteString(err.Error())
+		return
+	}
+
+	ctx.Writef("Received: %#+v", p)
+}
+
+func main() {
+	app := newApp()
+	app.Run(iris.Addr(":8080"))
+}