httputil: Fix incorrect Content-Type header

utils/httputil/client.go

@@ -140,8 +140,6 @@ func (c *Client) FastRequest(method, url string, opts ...RequestOption) error {
 
 // RequestJSON performs a request and unmarshals the JSON body into "to".
 func (c *Client) RequestJSON(to interface{}, method, url string, opts ...RequestOption) error {
-	opts = PrependOptions(opts, JSONRequest)
-
 	r, err := c.Request(method, url, opts...)
 	if err != nil {
 		return err

utils/httputil/httpdriver/default.go

@@ -72,7 +72,7 @@ func (r *DefaultRequest) AddQuery(values url.Values) {
 
 func (r *DefaultRequest) AddHeader(header http.Header) {
 	for key, values := range header {
-		r.Header[key] = append(r.Header[key], values...)
+		r.Header[key] = values
 	}
 }