`Content-Type: application/json` is not set properly

[xuxinx]

Currently response text/plain, not application/json

What Golang rules:
https://github.com/golang/go/blob/9e277f7d554455e16ba3762541c53e9bfc1d8188/src/net/http/server.go#L94-L101

What jsonhandlerfunc do:

jsonhandlerfunc/handler.go

Lines 110 to 111 in cf20ea2

	w.WriteHeader(httpCode)
	writeJSONResponse(w, outs)

jsonhandlerfunc/handler.go

Lines 219 to 220 in cf20ea2

	func writeJSONResponse(w http.ResponseWriter, out interface{}) {
	w.Header().Set("Content-Type", "application/json")