attempting to add headers and add test

[Jacob-Eliat-Eliat]

I tried various approaches to test this feature, including:

• request with different headers to cognite.com => no way to know which headers were sent
• request to files with gzip disabled/enabled => seems like the file download endpoint doesn't compress, at least not for small files

In the end I gave up, but not before trying to see if the headers were correctly overriden using https://www.postman.com/postman/published-postman-templates/documentation/ae2ja6x/postman-echo?ctx=documentation and they are so this should be working right... but not sure how to test it properly without refactor

[dmivankov]

note that headers from sttpRequest would be preserved if not present in argument, it makes sense by default for things like x-cdp-app, x-cdp-sdk etc, for .acceptEncoding we can still override it with headers, but if main motivation is only acceptEncoding then maybe add dedicated parameter for it?

[Jacob-Eliat-Eliat]

Maybe I can use "overrideHeaders" but that was my intention here. Just being a bit more general.