Use Content-Type header in HTTP requests

[jelcaf]

What is the expected feature or enhancement?

This header is essential for indicating the media type of the data being sent in the request body, especially when working with APIs that expect specific formats like JSON.

It seems that we tried to set that header as default here:

qiskit-ibm-runtime/qiskit_ibm_runtime/api/rest/base.py

Line 24 in 639f307

_HEADER_JSON_CONTENT = {"Content-Type": "application/json"}

But I think that it's not used.

Acceptance criteria

• Send Content-Type header as application/json if the request body is a dictionary or list, unless the user specifies a different value.