LP#1993754: fix XML-RPC gateway's handling of Fieldmapper objects

LP#1993754: fix XML-RPC gateway's handling of Fieldmapper objects

This patch improves the handling of Fieldmapper objects by the
XML-RPC gateway by:

- enabling support for the XML-RPC nil extension, thereby allowing
  null values to be correcty passed back and forth rather than
  converting them to empty strings
- fixing the de-serialization of Fieldmapper objects; this allows
  a Fieldmapper object returned by one XML-RPC call to be modified
  and consumed by a second one

To test
-------
[1] Apply the patch but do not restart Apache.
[2] Run the new test script test-xml-rpc-patron-update.pl to
    try update a patron record; note that the update fails.
[3] Restart or reload Apache and repeat step 2; this time, the
    patron update should succeed.

Note that the test script may need minor modification to work on your
local testing environment.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>