Change the type of the element/attribute for cacheDuration to a String

The `cacheDuration` in the SAML Metadata XML is [defined (line #349)](http://docs.oasis-open.org/security/saml/v2.0/saml-metadata-2.0-os.pdf) as a `Duration` data type, in the form of `PnYnMnDTnHnMnS`. To support this data type, the `cache_duration` attribute/element (is the same in `HappyMapper`) should be casted to a `String` instead of an `Integer`.

To be honest, I have no idea how to test this properly, maybe someone else can provide the specs?