Xslt: disabled ranges.

Xslt: disabled ranges.

Previously, the document generated by the xslt filter was always fully sent
to client even if a range was requested and response status was 206 with
appropriate Content-Range.

The xslt module is unable to serve a range because of suspending the header
filter chain.  By the moment full response xml is buffered by the xslt filter,
range header filter is not called yet, but the range body filter has already
been called and did nothing.

The fix is to disable ranges by resetting the r->allow_ranges flag much like
the image filter that employs a similar technique.