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    http.setRequestHeader - define a custom request header
Syntax
http.setRequestHeader(header, value)
Parameters
http - is the variable name used in a prior call to new XMLHttpRequest;
header - string containing name of the header to set.
value - string value to set the header.
Returned Value
Zero if successful.
Description
Define a custom request header.
Example
 var http = new XMLHttpRequest;
 http.open("POST", "http://www.docorigin.com/echo.php");
 http.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
 var sData = "first=one&second=two";
 http.send(sData);
 var sResult = http.responseText;    // what was sent back
 _message("result='%s'", sResult);