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/xml.go

@@ -5,7 +5,9 @@
 package binding
 
 import (
+	"bytes"
 	"encoding/xml"
+	"io"
 	"net/http"
 )
 
@@ -16,7 +18,14 @@ func (xmlBinding) Name() string {
 }
 
 func (xmlBinding) Bind(req *http.Request, obj interface{}) error {
-	decoder := xml.NewDecoder(req.Body)
+	return decodeXML(req.Body, obj)
+}
+
+func (xmlBinding) BindBody(body []byte, obj interface{}) error {
+	return decodeXML(bytes.NewReader(body), obj)
+}
+func decodeXML(r io.Reader, obj interface{}) error {
+	decoder := xml.NewDecoder(r)
 	if err := decoder.Decode(obj); err != nil {
 		return err
 	}