Content-Type duplication in POST requests

[DiogoP98]

Screenshot

Description

When making a POST request with a content-type header, a new Content-Type header is added to the request because the map is not case insensitive. This is leading to MIME erros when querying some APIs.
This goes against the best-practices as headers' field names should be case insensitive.

Ref: https://www.rfc-editor.org/rfc/rfc2616.html#section-4.2:~:text=Field%20names%0A%20%20%20are%20case%2Dinsensitive

Reproduction Steps/Repo Link

• Make a POST request with a JSON body and a content-type header
• The new header Content-Type will be added

[jonkoops]

This is leading to MIME erros when querying some APIs.

Note that this is also a bug in these APIs, as they also should treat headers in a case-insensitive manner. Still, this should probably be corrected on the side of the client.