No support for `content` property - only `contentMap` works

[warriordog]

These properties are related and should be handled together. Currently, we ignore contentMap and incorrectly handle content. It should work like this:

• contentMap converts directly to/from ContentMap
• content converts indirectly to/from ContentMap.DefaultValue
• We only store one backing field - the other is just an alias.

Once resolved, then the unit test MastodonUpdateNoteActivityTest should finally work.

Depends on: #12 Relates to: #44