net-http: Allow symbol based hashes for headers

[ThisGuyCodes]

These methods work fine with headers given as Hash[Symbol, untyped]. I noticed when I got some squiggly lines in some of my methods that I already know work. I also defined headers as a type alias rather than copy-pasting the same definition everywhere (I can undo this if not desired).

I did (Hash[String, untyped] | Hash[Symbol, untyped]) rather than Hash[String | Symbol, untyped] because hashes with mixed key types can allow name collisions which gives unexpected behavior.

I also feel like we can do better than untyped for the values, but leaving it for now.