Preserve original Content-Type in OHTTP requests

[D13410N3]

Current behaviour of gocurl explicitly sets "Content-Type", "application/x-www-form-urlencoded" if request data is not empty

func addBodyHeaders(req *http.Request, cfg *config.Config) {
	if cfg.Data != "" && !websocket.IsWebSocket(cfg.RequestURL) {
		req.Header.Add("Content-Type", "application/x-www-form-urlencoded")

If user's request contains its own "Content-Type" header - both headers are clued to one

...
"Content-Type": "application/x-www-form-urlencoded  application/json",

This is wrong behaviour - we need to overwrite user's request content-type

This PR fix this behaviour