CompiledExpressions - convertTypeString

When testing types using expressions the use of unsignedLong failed with a feature not yet implemented message. This is due to the fact that certain return types (ulong is an example) are not implemented in XPath 1.0.

http://saxonica.com/documentation9.4-demo/html/xpath-api/jaxp-xpath/return-types.html

test_contentLength_0 is an example of this failure.

This need to be fixed and tests need to be created for this.