support bind http header param #1956 (#1957)

* support bind http header param #1956

update #1956
```
package main

import (
	"fmt"
	"github.com/gin-gonic/gin"
)

type testHeader struct {
	Rate   int    `header:"Rate"`
	Domain string `header:"Domain"`
}

func main() {
	r := gin.Default()
	r.GET("/", func(c *gin.Context) {
		h := testHeader{}

		if err := c.ShouldBindHeader(&h); err != nil {
			c.JSON(200, err)
		}

		fmt.Printf("%#v\n", h)
		c.JSON(200, gin.H{"Rate": h.Rate, "Domain": h.Domain})
	})

	r.Run()

// client
// curl -H "rate:300" -H "domain:music" 127.0.0.1:8080/
// output
// {"Domain":"music","Rate":300}
}
```

* add unit test

* Modify the code to get the http header

When the http header is obtained in the standard library,
the key value will be modified by the CanonicalMIMEHeaderKey function,
and finally the value of the http header will be obtained from the map.
As follows.
```go
func (h MIMEHeader) Get(key string) string {
        // ...
         v := h[CanonicalMIMEHeaderKey(key)]
        // ...
}
```

This pr also follows this modification

* Thanks to vkd for suggestions, modifying code

* Increase test coverage

env GOPATH=`pwd` go test github.com/gin-gonic/gin/binding -coverprofile=cover.prof
ok  	github.com/gin-gonic/gin/binding	0.015s	coverage: 100.0% of statements

* Rollback check code

* add use case to README.md

README.md

@@ -40,6 +40,7 @@ Gin is a web framework written in Go (Golang). It features a martini-like API wi
     - [Only Bind Query String](#only-bind-query-string)
     - [Bind Query String or Post Data](#bind-query-string-or-post-data)
     - [Bind Uri](#bind-uri)
+    - [Bind Header](#bind-header)
     - [Bind HTML checkboxes](#bind-html-checkboxes)
     - [Multipart/Urlencoded binding](#multiparturlencoded-binding)
     - [XML, JSON, YAML and ProtoBuf rendering](#xml-json-yaml-and-protobuf-rendering)
@@ -910,6 +911,43 @@ $ curl -v localhost:8088/thinkerou/987fbc97-4bed-5078-9f07-9141ba07c9f3
 $ curl -v localhost:8088/thinkerou/not-uuid
 ```
 
+### Bind Header
+
+```go
+package main
+
+import (
+	"fmt"
+	"github.com/gin-gonic/gin"
+)
+
+type testHeader struct {
+	Rate   int    `header:"Rate"`
+	Domain string `header:"Domain"`
+}
+
+func main() {
+	r := gin.Default()
+	r.GET("/", func(c *gin.Context) {
+		h := testHeader{}
+
+		if err := c.ShouldBindHeader(&h); err != nil {
+			c.JSON(200, err)
+		}
+
+		fmt.Printf("%#v\n", h)
+		c.JSON(200, gin.H{"Rate": h.Rate, "Domain": h.Domain})
+	})
+
+	r.Run()
+
+// client
+// curl -H "rate:300" -H "domain:music" 127.0.0.1:8080/
+// output
+// {"Domain":"music","Rate":300}
+}
+```
+
 ### Bind HTML checkboxes
 
 See the [detail information](https://github.com/gin-gonic/gin/issues/129#issuecomment-124260092)