AJAX 异步传输数据的问题

要异步传输的数据：
Xml代码
....
<action xsi:type="basic:JavaScript" script="index += 1;"/>
....
Ajax异步传输代码：
Js代码

代码如下：

var postData = "input="+ escape(inputJSON) +"&script="+escape(xml)+ 
"&feedGeneral=" + escape(feedGeneral); 
XmlHttpRequest.open("POST",url,true); 
XmlHttpRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded"); 
XmlHttpRequest.send(postData);

postData在encode和unencode，最终导致在后台Servlet中得到得到数据+被空格代替，使得script中的index += 1;变成了index = 1;从而导致后台Java代码在跑script出现死循环。
在网上搜索，发现content-type使用application/x-www-form-urlencoded后：
[来自http://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.1]写道

代码如下：

Control names and values are escaped. Space characters are replaced by `+', and then reserved characters are escaped as 
described in [RFC1738], section 2.2: Non-alphanumeric characters are replaced by `%HH', a percent sign and two hexadecimal 
digits representing the ASCII code of the character. Line breaks are represented as "CR LF" pairs (i.e., `%0D%0A').

然而使用form来提交方式来发起request却不会出现类似的问题，而form默认的Content-Type也是application/x-www-form-urlencoded:
Js代码

代码如下：

$('test').innerHTML = "<form target='_blank' id='test_form' action='./gen_feed' method='post'>" 
+ "<input type='text' name='input' /><input type='text' name='script' />" 
+ "<input type='text' name='feedGeneral' /><input type='hidden' name='format' value='" + this.feed_type + "' 
/>" 
+ "<input type='submit' value='gen' /></form>"; 
var test_form = $('test_form'); 
test_form.elements[0].value = inputJSON; 
test_form.elements[1].value = script; 
test_form.elements[2].value = feedGeneral; 
test_form.submit();

仍未发现问题到底出在何处，暂做备忘。暂时把script中的‘+'都用‘-'代替，index += 1;改成index -= -1;呵呵，以后有人看到这段自动生成的诡异脚本，不知道会作何感想，但现在也只能如此。