[TIMOB-13555] MobileWeb: Cannot change Content-Type passing an object in the send() method of Ti.Network.HTTPClient

Problem description

When trying to change the Content-Type of the HTTPClient request (e.g. to set it to application/json), the header sent still contains the default application/json,application/x-www-form-urlencoded value, in addition to what was set.

Steps to reproduce

Use the following code:

var win = Ti.UI.createWindow();
win.open();

var xhr = Ti.Network.createHTTPClient({
	onload : function(e) {
		Ti.API.info('onload called, readyState = ' + this.readyState);
	},
	onerror : function(e) {
		Ti.API.info('onerror called, readyState = ' + this.readyState);
	},
	timeout : 30000 /* in milliseconds */
});
xhr.open("POST", 'http://127.0.0.1:8020/');
xhr.setRequestHeader("Content-Type", "application/json");
xhr.send({
	"CountryCode" : "ITA"
});

When running the code in MobileWeb, the Content-Type header will contain: Content-Type: application/json, application/x-www-form-urlencoded

Additional notes

According to the XMLHttpRequest specifications, setRequestHeader will add the request header when called, and not replace it. Apparently, "application/x-www-form-urlencoded" is always set if any data is sent.