The connection content_type should be modified based on the type of operation occuring

[trevor-vaughan]

I was attempting to use this gem with podman and I'm not sure if it has more strict content type checks than docker, but the default that sets application/json appears to be incorrect for various operations based on the API docs.

https://github.com/swipely/docker-api/blob/1e9b9cc5f0f38dcd54c18189812328bb802d3656/lib/docker/connection.rb#L79