Cannot drop a record property

[n1k0]

Because we perform PATCH requests, the sent record payload is always merged with its server version, so the property value always remains.

We could merge form values into initial record object when sending the request.

As patching is only required for collection attributes, so we don't lose attributes not handled by the edit form, we should:

1. Drop our use of the patch option for updating buckets, groups and records (so we always replace records);
2. Update the update logic for collections to always preserve existing fields when they initially exist on the server.

Spawned from #377.

[n1k0]

Fixed by #379.