Set subject values to nil when needed

[mreq]

Set values to nil when not present in XML.

Steps to reproduce (before applying this fix):

vat_paying_subject = RubyARES::Subject.get(27074358)
non_vat_subject = RubyARES::Subject.get(non_vat_number)

vat_paying_subject.dic == non_vat_subject.dic
=> true

Previously, subsequent calls would keep the values of previous calls when not present in new XML.