For associations where we expect only one record, if there are multiple, then we should raise an error

[rpbaltazar]

E.g. Has One Resource assumes that there is only one associate resource. If for some reason the data got out of sync, we'd be happily filtering out other results instead of explaining that there are inconsistent data.

[rpbaltazar]

We should probably add an association error class to handle these cases.
E.g. AssociationError::CountMismatch