Allow multiple NameIDFormats for IdPs

The SAML2int spec suggests that IdPs should advertise two name
identifier formats, and SAML itself supports this. It seems that
SimpleSAMLphp does too, when handling metadata in XML (it is implemented
as an array). However the internal metadata format uses getString,
limiting us to only a single NameIDFormat. So far as I can tell, all
that's needed to fix this is to change the metadata parser to use
getArrayizeString to accept either a string or an array, and to cast
that as a string when necessary.
This may solve issue simplesamlphp/simplesamlphp#91