Ensure specification validation checks relations are of expected type (to-one or to-many)

[lindyhopchris]

RE: this issue:
cloudcreativity/laravel-json-api#335

We need to ensure that the specification validator rejects a document that has the wrong type of relation - e.g. submitting a to-many for a to-one, and vice versa.

The spec validator now has access to this information via the schema classes.

I suspect I've already implemented this, but just need to double-check that it is implemented and tested before closing this issue!

[lindyhopchris]

The spec library is rejecting a to-one for a to-many relation, and vice versa, but the message isn't great. For to-one it says the data member should be an object; for a to-many it says the member should be an array.

Would be an improvement if it rejected saying: The field author must be a to-one relation. and The field tags must be a to-many relation.

[lindyhopchris]

Will be released in the spec package 1.0.0-alpha.3