Fix #216: Enable to call binding multiple times in some formats (#1341)

* Add interface to read body bytes in binding

* Add BindingBody implementation for some binding

* Fix to use `BindBodyBytesKey` for key

* Revert "Fix to use `BindBodyBytesKey` for key"

This reverts commit 2c82901ceab6ae53730a3cfcd9839bee11a08f13.

* Use private-like key for body bytes

* Add tests for BindingBody & ShouldBindBodyWith

* Add note for README

* Remove redundant space between sentences

binding/msgpack.go

@@ -5,6 +5,8 @@
 package binding
 
 import (
+	"bytes"
+	"io"
 	"net/http"
 
 	"github.com/ugorji/go/codec"
@@ -17,7 +19,16 @@ func (msgpackBinding) Name() string {
 }
 
 func (msgpackBinding) Bind(req *http.Request, obj interface{}) error {
-	if err := codec.NewDecoder(req.Body, new(codec.MsgpackHandle)).Decode(&obj); err != nil {
+	return decodeMsgPack(req.Body, obj)
+}
+
+func (msgpackBinding) BindBody(body []byte, obj interface{}) error {
+	return decodeMsgPack(bytes.NewReader(body), obj)
+}
+
+func decodeMsgPack(r io.Reader, obj interface{}) error {
+	cdc := new(codec.MsgpackHandle)
+	if err := codec.NewDecoder(r, cdc).Decode(&obj); err != nil {
 		return err
 	}
 	return validate(obj)