Fix: Compilation error on XSLT with includes.
Fix: Compilation error on XSLT with includes.

A null XmlUrlResolver was being passed to Load() which prevented
includes from being processed. Fixed by passing a new XmlUrlResolver(),
but we still pass null when UseTrusted==true because external XSLT files
are not trusted.

stimpy77/msbuildtasks,loresoft/msbuildtasks,jricke/msbuildtasks,AArnott/msbuildtasks,wrozmiarek/msbuildtasks,smallkid/msbuildtasks,arekbee/msbuildtasks