Default Caching HTTP Headers in xmlhttp

I am using xmlhttp to retrieve a very large static xml file. I really 
want this to be cached, so that the load time of my application is as 
small as possible, however Mozilla's default HTTP Headers seem to be 
made to frustrate this effort, by switching off any caching. .

See the code below and a description of its results for more details.
___________________________________________

xmlhttp = new XMLHttpRequest();
	
xmlhttp.open("GET", "huge.xml",true);
xmlhttp.onreadystatechange=function() {
	if (xmlhttp.readyState==4) {
		alert(xmlhttp.responseText);
	}
}

xmlhttp.send(null);

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This code will produce a GET Request with the following HTTP headers: 
Cache-Control: no-cache
Pragms: no-cache

This disables any caching. I don't want this at all. I have tried to 
override the values using commands like.

	xmlhttp.setRequestHeader('Cache-Control', '');
	xmlhttp.setRequestHeader('Pragma', '');

This has no effect at all.

Another attempt:

xmlhttp.setRequestHeader('Cache-Control', 'Hello');
xmlhttp.setRequestHeader('Pragma', 'Hello');

Produces the following HTTP Headers:
Cache-Control: Hello, no-cache
Pragms: Hello, no-cache

Which still seems to override any ability to use the caching data.

If anybody has any useful ideas or explanation for this behavior please 
don't hesitate to contact me.

Kind regards,

Jeremy Hartley