fix: don't convert header names to lowercase

[jgiovaresco]

I've completed @darrenjennings branch to remove the .toLowerCase for header names.

Closes#74

[develohpanda]

Thanks for finishing this off!

Do you have any thoughts about whether handling headers with HTTP/1.1 (case-insensitive) vs HTTP/2 (case-sensitive) is the responsibility of this library, or responsibility of the consumer? (cc: @darrenjennings)

I am impartial, but since we have access to the version on the HAR object, we might as well follow the RFC.

[develohpanda]

Looks good now, nice to see an additional unit test! 👍

Just a minor comment on the use of regex

[develohpanda]

Nicely done 🙌

[reynolek]

released in 1.23.0

[erunion]

This change has unfortunately introduced some header duping in some cases of commonly used headers like Content-Type where if it's as Content-Type in the HAR, but a target is looking for content-type or adds its own content-type header (like in multipart/form-data where it is overridden) we end up with double headers containing potentially differing values.

I'm working on a PR right now though.