Add author copyrights to the header map file

[bluss]

The HeaderMap implementation as a whole is mostly orignal to http-rs, but the implementation was based on and derived from the indexmap crate (formerly ordermap) from circa 2017, from which the structure and many methods were adapted. For this reason, mention the two indexmap authors (one of them is me) from that time.

Fixes #489

[bluss]

Any other solution is fine to me too, just making a draft. How to mention non-indexmap authors I leave up to you. It's by no means an easy question. But I guess the point here is that we're making an extra effort to mention the indexmap authors - since they are invisible in the git history and contributor list of the repo.