When the `required` and `optional` qualifiers on `belong_to` fail, the
message can be a little confusing. There are actually two ways to
satisfy both qualifiers. One way, of course, is to match the option on
the association with the qualifier itself, so if the association is
defined with `required: true`, that will match
`belong_to(...).required`, and if it's `required: false`, that will
match `belong_to(...).required(false)` (and similar for `optional`). The
second way, though, is to manually add a presence validation to the
association (this will obviously only match `required(true)`). We need
to update the failure message to reflect these cases.
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