Remove typename from POI

[evaporei]

After an entity merge, if the Entity Cache was cleared, the __typename
field would come up again and polute mappings and by consequence, the
POI. This PR fixes the issue/bug alongside with it's respective (new) test.

The merging behavior is described here in the docs (second item).

### Updating existing entities
There are two ways to update an existing entity:

1. Load the entity with e.g.`Transfer.load(id)`, set properties on the entity, then `.save()` it back to the store.
2. Simply create the entity with e.g. `new Transfer(id)`, set properties on the entity, then `.save()` it to the store. If the entity already exists, the changes are merged into it.

[leoyvens]

Nice work with the test!