- Working with Conn APIs requires calling new/0 or new/1 first
  old APIs are deprecated
- new/1 parses the provided string with URI.parse/1 and sets the Conn
  struct fields accordingly (so you can pass full or partial url
  fragments and get what you would expect, including those containing
  query strings)
- All headers are normalized to lower-case on put/get, and internally
  headers are a simple key/value map instead of the previous key =>
  key/value map, which was unnecessary.
- Implemented get_req_headers/1, deprecated get_req_header/1
- Implemented put_req_headers/2, deprecated put_req_header/2
- get_req_header/2 returns String.t instead of {String.t, String.t}
- get_req_headers/1 returns %{String.t => String.t} instead of
  %{String.t => %{String.t => String.t}}
- Added a number of typespecs, and fixed invalid ones
- Some small style fixes, documentation updates
- Fixed a bug with the Header middleware which was not overriding
  headers in the connection if already set. This one may be up for
  discussion if what's really desired.
- Added HeaderCase middleware for enforcing header casing

- Added more tests