v0.5.0 Change headers type to binary and preserve headers case

Changes

• Lowercase method atoms
• Use binaries both for header names and values
• Introduce raw_headers that preserve headers case and order. erlang/otp#2466 allows keeping headers case
• Some documentation updates
• Update deps

Breaking changes

• bookish_spork_request:headers/1 now returns a map with binaries for both keys and values instead of erlang stings
• bookish_spork_request:header/2 returns binary instead of string