Gin gin.CreateTestContext is not working

Description

When I try to use gin.CreateTestContext(), it create a context that cannot be used to serve http requests. Indeed, it takes a http.ResponseWriter as a parameter, which is incompatible with http.CloseNotifier, causing a panic.

How to reproduce

func Test(t *testing.T) {
    c, _ := gin.CreateTestContext(httptest.NewRecorder())

    request, err := http.NewRequest("GET", "/ping", nil)
    if err != nil {
        panic(err)
    }

    url, _ := url.Parse("http://localhost:8080")
    proxy := httputil.NewSingleHostReverseProxy(url)

    proxy.ServeHTTP(c.Writer, request)
}

Expectations

gin.CreateTestContext() should take as parameter an interface compatible with the http.CloseNotifier interface and should not produce a panic.

Actual result

The code panics.

--- FAIL: Test (0.00s)
panic: interface conversion: *httptest.ResponseRecorder is not http.CloseNotifier: missing method CloseNotify [recovered]
    panic: interface conversion: *httptest.ResponseRecorder is not http.CloseNotifier: missing method CloseNotify

goroutine 35 [running]:
testing.tRunner.func1.2(0xa79ba0, 0xc0003838f0)
    /home/linuxbrew/.linuxbrew/Cellar/go/1.16.6/libexec/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc0003c0300)
    /home/linuxbrew/.linuxbrew/Cellar/go/1.16.6/libexec/src/testing/testing.go:1146 +0x4b6
panic(0xa79ba0, 0xc0003838f0)
    /home/linuxbrew/.linuxbrew/Cellar/go/1.16.6/libexec/src/runtime/panic.go:965 +0x1b9
github.com/gin-gonic/gin.(*responseWriter).CloseNotify(0xc00040c300, 0xc000126010)
    /home/paul/go/pkg/mod/github.com/gin-gonic/gin@v1.7.3/response_writer.go:112 +0x45
net/http/httputil.(*ReverseProxy).ServeHTTP(0xc000398780, 0x7f853e3e8970, 0xc00040c300, 0xc00040c400)
    /home/linuxbrew/.linuxbrew/Cellar/go/1.16.6/libexec/src/net/http/httputil/reverseproxy.go:223 +0x1693
gitlab.enix.io/products/docker-cache-registry/internal/proxy.Test(0xc0003c0300)
    /home/paul/dev/internal/docker-cache-registry/internal/proxy/server_test.go:38 +0x1b1
testing.tRunner(0xc0003c0300, 0xb5bc48)
    /home/linuxbrew/.linuxbrew/Cellar/go/1.16.6/libexec/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
    /home/linuxbrew/.linuxbrew/Cellar/go/1.16.6/libexec/src/testing/testing.go:1238 +0x2b3
FAIL    gitlab.enix.io/products/docker-cache-registry/internal/proxy    0.007s
FAIL
 ```

## Environment

- go version: go1.16.6 linux/amd64
- gin version (or commit ref): v1.7.3
- operating system: ubuntu 20.04

## Notes

The issue is coming from `responseWriter.CloseNotify()`:

// CloseNotify implements the http.CloseNotify interface. func (w *responseWriter) CloseNotify() <-chan bool { return w.ResponseWriter.(http.CloseNotifier).CloseNotify() } ```

w.ResponseWriter is casted without verification to http.CloseNotifier. However, the httptest.ResponseRecorder interface returned by gin.CreateTestContext() is incompatible with http.CloseNotifier.

Comment From: myzhan

Yes, I met the same issue when using gin as a reverse proxy.