format DateTime according to xsd:dateTime

 XML Schema defines a usage profile
 which is a subset of the ISO8601 profile.
 This profile defines that the following
 is the only possible representation for a dateTime,
 despite all other options ISO provides.
 .
  YYYY-MM-DD"T"HH:MI:SS(Z|[+-]zh:zm)
 .
 DateTime::Format::XSD is a subclass of DateTime::Format::ISO8601,
 therefore it will be able to parse all other ISO options,
 but will only format it in this exact spec.