Fix Context.get/put in LiveComponent update

[tiagoefmoraes]

Allow calling Context.get(socket, :field) and Context.put(socket, field: "value") in update/2

Calling it giving assigns will not work because there's no context information there. It's different from a render/1 assigns.

If the user is defining props or datas with from_context they will have to call assign(socket, assigns) for them to be updated (as with any other assigns).

[msaraiva]

@tiagoefmoraes thank you!