code enhancement

[cybersky]

WXPay.mix('getBrandWCPayRequestParams', function(order, fn){

order.trade_type = "JSAPI";
var _this = this;
this.createUnifiedOrder(order, function(err, data){

//TODO: check data['return_code'] and data['return_msg'] response here, otherwise client will get a callback data with like {..., nonceStr: undefined, package: "prepay_id=undefined"... } without any further debug infomation.

	var reqparam = {
		appId: _this.options.appid,
		timeStamp: Math.floor(Date.now()/1000)+"",
		nonceStr: data.nonce_str,
		package: "prepay_id="+data.prepay_id,
		signType: "MD5"
	};
	reqparam.paySign = _this.sign(reqparam);
	fn(err, reqparam);
});

});