Warn on use of Mapper without SaveBehavior.CLOBBER

It is important to use SaveBehavior.CLOBBER with AttributeEncryptor. If you do not do so you risk corrupting your signatures and encrypted data.

When CLOBBER is not specified, fields that are present in the record may not be passed down to the encryptor, which results in fields being left out of the record signature. This in turn can result in records failing to decrypt.

This version adds an ERROR-level log message to warn users that are at risk. A future version bump will introduce an exception in this case.

See also issue #32