LP#1442276 pgTAP test for oils_xslt_process

LP#1442276 pgTAP test for oils_xslt_process

In moving from the deprecated XML::LibXSLT::output_string() to output_as_chars(),
add a regression test to ensure that we do not suffer from corrputed encoding
output in the future.

We test both the case where an output encoding has been explicitly declared,
as well as the case where an output encoding has not been explicitly declared.
It was this subtle difference that was causing the problem with output_string().

Signed-off-by: Dan Scott <dan@coffeecode.net>