Status.filtered is not null for unfiltered statuses

[bocops]

Documentation states that Status.filtered is optional with the definition

If the current token has an authorized user: The filter and keywords that matched this status.

This is why I implemented this as a nullable value of the Status entity. However, my own use of the filter API seems to suggest that this is actually never not returned with individual statuses. I assumed that I could just test if filtered is non-null to determine that a status is affected by filtering, but this is wrong.

If a status is unaffected by any filters, this will be an empty array "[]" in the returned Json. This can be checked with slight changes to the sample I just provided with #221. I am unsure if we should consider this a bug in Mastodon's documentation, or if we should just add better documentation on our side.