Add attributes from the original (heras)obligation into the opensaml XACML obligation

The original org.herasaf.xacml.core.policy.impl.ObligationType must be fully copied into the
org.opensaml.xacml.policy.ObligationsType, i.e. including the attributes.
Note that the latter obligation type does not support multi-valued attributes (it only has
setValue(String)) so we warn and just take the first value.
For the datatype we use getDatatypeURI().