This method can be called with an array, or a relation:

1. Arrays always have a limited amount of values, so that's fine.
2. If the relation does not have a limit value applied, then we will load every
   single object in that collection, and prevent N+1 queries for the metadata
   for that. But that's wrong, because we should never call this without an
   explicit limit set. So we raise in that case, and this commit will see which
   specs fail.

The only failing specs here were the issues API specs, and the specs for
IssuableMetadata itself, and both have been addressed.