feat: add GetRawData func. (#857)

* feat: add GetRawData func.

* update return style

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

context.go

@@ -7,6 +7,7 @@ package gin
 import (
 	"errors"
 	"io"
+	"io/ioutil"
 	"math"
 	"mime/multipart"
 	"net"
@@ -445,6 +446,11 @@ func (c *Context) GetHeader(key string) string {
 	return c.requestHeader(key)
 }
 
+// GetRawData return stream data
+func (c *Context) GetRawData() ([]byte, error) {
+	return ioutil.ReadAll(c.Request.Body)
+}
+
 func (c *Context) SetCookie(
 	name string,
 	value string,