Azure-requests

- Better error message for 4xx HTTP codes (e.g. expired access token)

- Return the plain `requests` response if it is not a JSON content.
This allows to download attachments, for example.

- Fix bug: work items cannot be created due to wrong content-type (Now use json
patch content type if the payload is an array, not just for PATCH HTTP method)

- Add `call` method to have unified simple interface for all API endpoints

- Retry on proxy error

- Fix Content-Type header