Change ReferralMode.toString to return value

The documentation of the referral-mode attribute in WildFly says the allowed values should be "ignore", "follow", "throw". However they are currently serialized as "IGNORE", "FOLLOW", "THROW". This is a WildFly issue and not an Elytron issue.
However as part of the discussion on how to best fix WFCORE-3971 the easiest solution seems to be to change ReferralMode#toString(), that way the parser does not have to change.
Once WildFly upgrades to a version of Elytron with this change a couple of tests will fail as the serialized model will intentionally change.