Since an unchecked HTML checkbox has no value at all, a hidden field may be
used to specify an off value for the checkbox.

Since the specs aren't 100% clear, dates might be passed in different
formats by various XMLRPC clients. This patch makes date parsing a bit
more flexible.

Unit tests included.

according to the specs a value without a type is
recognised as string. This patch handles empty value
tags the right way as empty String.

This patch fixes a bug introduced in »code cleanup« #0b17fdc6