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

README.md

@@ -52,6 +52,7 @@ Gin is a web framework written in Go (Golang). It features a martini-like API wi
     - [Graceful restart or stop](#graceful-restart-or-stop)
     - [Build a single binary with templates](#build-a-single-binary-with-templates)
     - [Bind form-data request with custom struct](#bind-form-data-request-with-custom-struct)
+    - [Try to bind body into different structs](#try-to-bind-body-into-different-structs)
 - [Testing](#testing)
 - [Users](#users--)
 
@@ -1554,6 +1555,64 @@ type StructZ struct {
 
 In a word, only support nested custom struct which have no `form` now.
 
+### Try to bind body into different structs
+
+The normal methods for binding request body consumes `c.Request.Body` and they
+cannot be called multiple times.
+
+```go
+type formA struct {
+  Foo string `json:"foo" xml:"foo" binding:"required"`
+}
+
+type formB struct {
+  Bar string `json:"bar" xml:"bar" binding:"required"`
+}
+
+func SomeHandler(c *gin.Context) {
+  objA := formA{}
+  objB := formB{}
+  // This c.ShouldBind consumes c.Request.Body and it cannot be reused.
+  if errA := c.ShouldBind(&objA); errA == nil {
+    c.String(http.StatusOK, `the body should be formA`)
+  // Always an error is occurred by this because c.Request.Body is EOF now.
+  } else if errB := c.ShouldBind(&objB); errB == nil {
+    c.String(http.StatusOK, `the body should be formB`)
+  } else {
+    ...
+  }
+}
+```
+
+For this, you can use `c.ShouldBindBodyWith`.
+
+```go
+func SomeHandler(c *gin.Context) {
+  objA := formA{}
+  objB := formB{}
+  // This reads c.Request.Body and stores the result into the context.
+  if errA := c.ShouldBindBodyWith(&objA, binding.JSON); errA == nil {
+    c.String(http.StatusOK, `the body should be formA`)
+  // At this time, it reuses body stored in the context.
+  } else if errB := c.ShouldBindBodyWith(&objB, binding.JSON); errB == nil {
+    c.String(http.StatusOK, `the body should be formB JSON`)
+  // And it can accepts other formats
+  } else if errB2 := c.ShouldBindBodyWith(&objB, binding.XML); errB2 == nil {
+    c.String(http.StatusOK, `the body should be formB XML`)
+  } else {
+    ...
+  }
+}
+```
+
+* `c.ShouldBindBodyWith` stores body into the context before binding. This has
+a slight impact to performance, so you should not use this method if you are
+enough to call binding at once.
+* This feature is only needed for some formats -- `JSON`, `XML`, `MsgPack`,
+`ProtoBuf`. For other formats, `Query`, `Form`, `FormPost`, `FormMultipart`,
+can be called by `c.ShouldBind()` multiple times without any damage to
+performance (See [#1341](https://github.com/gin-gonic/gin/pull/1341)).
+
 ## Testing
 
 The `net/http/httptest` package is preferable way for HTTP testing.