Check whether request context is done before write

[WindzCUHK]

This PR is to avoid a panic when writing response.

issue

When the HTTP connection is closed, the context of the request ends. Any following write action to the response writer will cause the program to panic.

solution

Check request context before the actual write.
The current solution is try to fix the problem in the simplest way. After the context checking, there may be a small chances that the context is closed before the writing action and causes panic again.

stack trace:

panic: Header called after Handler finished
goroutine 1366 [running]:
net/http.(*http2responseWriter).Header(0xc0003be038, 0xc0003c4140)
        /usr/local/go/src/net/http/h2_bundle.go:6158 +0x99
github.com/yahoo/k8s-athenz-webhook.writeJSON(0xc07c00, 0xc0004a4e10, 0xc05cc0, 0xc0003be038, 0xa14d80, 0xc0003c4140)
        /go/pkg/mod/github.com/yahoo/k8s-athenz-webhook@v0.0.0-20190725182459-949d9ed74720/http.go:26 +0x1cb
github.com/yahoo/k8s-athenz-webhook.(*authorizer).ServeHTTP(0xc000212280, 0xc05cc0, 0xc0003be038, 0xc0006cc600)
        /go/pkg/mod/github.com/yahoo/k8s-athenz-webhook@v0.0.0-20190725182459-949d9ed74720/authz.go:286 +0x47f
github.com/yahoo/k8s-athenz-webhook.wrapHandler.func2(0xc05cc0, 0xc0003be038, 0xc0006cc400)
        /go/pkg/mod/github.com/yahoo/k8s-athenz-webhook@v0.0.0-20190725182459-949d9ed74720/http.go:88 +0x14f
net/http.HandlerFunc.ServeHTTP(0xc00012c6c0, 0xc05cc0, 0xc0003be038, 0xc0006cc400)
        /usr/local/go/src/net/http/server.go:2007 +0x44
github.com/yahoojapan/garm/service.(*athenz).AthenzAuthorizer(0xc00014b0e0, 0xc05cc0, 0xc0003be038, 0xc0006cc400, 0xc0001fa1c0, 0xc05cc0)
        /go/src/github.com/yahoojapan/garm/service/athenz.go:88 +0x58
github.com/yahoojapan/garm/handler.(*handler).Authorize(0xc000289010, 0xc05cc0, 0xc0003be038, 0xc0006cc400, 0xc0003be001, 0xc000120800)
        /go/src/github.com/yahoojapan/garm/handler/handlers.go:59 +0x55
github.com/yahoojapan/garm/router.routing.func1.1(0xc0001250e0, 0xc000239120, 0xc05cc0, 0xc0003be038, 0xc0006cc300, 0xc07bc0, 0xc0006547e0)
        /go/src/github.com/yahoojapan/garm/router/router.go:81 +0xed
created by github.com/yahoojapan/garm/router.routing.func1
        /go/src/github.com/yahoojapan/garm/router/router.go:79 +0x491

similar issue: kubernetes/kubernetes#29001

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