Test PatchUpdateCredentials on Pleroma

[billstclair]

I tested PatchUpdateCredentials against gab.com, which makes it very likely that it will work for a standard Mastodon server. It needs to be tested there and for a Pleroma server.

It would also be nice to update the Entity storage to identify that the profile metadata (Source.fields) does not exist. That should become Maybe (List Field) instead of just List Field. I think the JSON is distinguishable. The example should not have a UI for the fields if they are not returned by server.

Compare other differences between Mastodon and Pleroma account information, and update for those (in particular, Pleroma's JSON includes "pleroma" properties for the Account and Source entities).