PATCH routes: support array types

[mattwiebe]

The fields_attributes in api/v1/update_credentials are submitted in a multipart/form-data hash like:

# Simplifying to not show the boundary
fields_attributes[0][name]
Some Title
fields_attributes[0][value]
Some Value
fields_attributes[1][name]
Another Title
# etc

We were only handling the simple value type before. While we shouldn't need the other array types, I figured I'd be completionist.

This is required for Extra Fields editing in ActivityPub: https://github.com/Automattic/wordpress-activitypub/pull/788