Bug 17807: Add format parameter to XSLT_Handler->transform

Bug 17807: Add format parameter to XSLT_Handler->transform

Format may be chars (default), bytes or xmldoc.
Note: xmldoc is a XML::LibXML document object.
Since the default is chars, this does not affect current use.

Note: The format parameter (xmldoc) will be used later in one of the OAI
modules to prevent duplicated xml parsing.

Test plan:
Run t/db_dependent/XSLT_Handler.t

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>