Content-Type header not normalized before checking, causing duplication when manually set

[noushinfl]

When using @apollo/datasource-rest @6.4.1 the content-type header is conditionally set if not already present. However, the check only looks for 'content-type' in lowercase, while manual settings like 'Content-Type' (capitalized, as commonly written) are not recognized.
This results in two Content-Type headers being sent in the request — one manually set (Content-Type) and one automatically set (content-type), leading to duplicated headers.

Expected Behavior:
The RESTDataSource should normalize existing header keys (case-insensitively) before deciding whether to set content-type.

Impact:

Breaks strict HTTP servers or proxies expecting one Content-Type header

Causes unexpected behaviors in backend systems (e.g., parsing issues)

[Markus-ipse]

I spent a day trying to figure out why my Hono server failed when calling from my Apollo Server, but was fine when curling it.. It was due to the duplicate content-type header.