XML exchange only creates external variables from application headers that are strings

When using an XQuery expression such as

declare variable $control external;
$control mod 2 = 0

the XML exchange currently only creates external XQuery variables from strings. For example, if I set an application header to 4, the XML exchange does not create an external variable for it. On the other hand, if I set the header to "4" (a string), the XML exchange will create an external variable for it.

It would be useful if the XML exchange supported other application header data types.

I tested this on the latest code from trunk, approx. r1033046.