Make XSL filter transform the item, not the layout

Previously the XSL filter used the value of the "content" parameter
passed to the "run" method as the content to transform. But when
invoked as a layout, filters are passed the content of the _layout_
in this parameter, not the content of the item. As this filter can
only be invoked as a layout it did not work as expected.

This change modifies the filter to use the value of the ":content"
parameter passed to the class at initialization as the content to
transform, consistent with how layouts are invoked. It also updates
the filter's test cases to reflect how the filter is actually used
and to ensure the correct input is being transformed.
Merge pull request #401 from simonsouth/bug/make-xsl-filter-process-item-content

Make XSL filter transform the item, not the layout - issue #399