Provide a HttpHeaders class for platform-independent use

[stevenroose]

The dart:io.HttpHeaders actually belongs in this library.

I'd prefer at least to have it available here as well, so that there is one available for platform-independent development.

[stevenroose]

Same goes for HttpStatus and ContentType

[nex3]

I'm not a fan of providing giant enums for string-based values like these. I find it rarely provides value on top of just writing the strings yourself, while producing a lot of API overhead and making the code harder to read. I understand the argument that they provide better static checking, but I think constants should be used for improving the readability of the code and correctness should generally be left to the tests, and constants here don't help the readability.

I will point out that http_parser has a cross-platform MediaType class that's used by this package.