XPTY0004 raised instead of XPST0003

[Paul J. Lucas]

For the query:

let $x := <msg>42</msg>
return $x contains text 42

Zorba returns:

</tmp/x.xq>:2,25: type error [err:XPTY0004]: "xs:integer": can not promote to type "xs:string"; raised at /Users/pjl/src/flwor/zorba/trunk-2/zorba/src/runtime/core/sequencetypes.cpp:437

However, the expected result of the full-text query FTWordsValue-IntegerLiteral.xq (which is equivalent to the query above for the purpose of raising the error) is error XPST0003.