Metadata: Add support for default metadata.

[pablinos]

The current implementation of get for the metadata returns the standard default of an empty string, or empty array, based on the $single parameter.

This change calls get_default_metadata so that any defaults added, along with the hooks, are respected.

[pablinos]

I've kept this as a draft for now as it needs some tests, and it might be worth combining it with #53 to make get_metadata behave more like production.

[leogermani]

Thanks for that.

This all makes sense, we just need to find some time to write some tests for both PRs and make sure we cover the behavior we want.

I'll try to do this as soon as I can

[szepeviktor]

Maybe someone jumps in and contributes unit tests.

[pablinos]

This all makes sense, we just need to find some time to write some tests for both PRs and make sure we cover the behavior we want.

Yes definitely. I'll try and make some time for it too. The PR I was working on that would have needed this change ended up going in a different direction, but it's probably still worth us getting the metadata API working as close to production as possible. If I (almost) needed it once, I'm sure it'll come up again!