api: Ensure all PUT/POST/PATCH requests use Content-Type: application/json

[marclop]

Overview

In the future the ESS API will require all requests being made against it to contain the Content-Type: application/json header.

I'm fairly sure that our SDK already does this but we should double check that's the case writing appropriate tests that ensure we're doing so.

[marclop]

it is indeed the case. I'm closing this issue