Remove encoders abstraction

Follow-up of !241 (merged)

APIStar used a layer of encoders to support sending request bodies as JSON, multipart/form-data or application/x-www-form-urlencoded. Django REST Framework, thus the Arkindex API, supports all of those, but the client tries JSON first and will fall back to the others only if the JSON encoder is explicitly removed or if it is not supported by an endpoint. All of our endpoints have and will have JSON request bodies, so the other encoders are never relevant.

We can thus remove all of arkindex.client.encoders and all the encoders= arguments being passed around in the Arkindex client, base client and HTTP transport, and just use the json= argument of requests to send JSON all the time.