Plug.Conn.req_headers

[WilliamTakeshi]

Hello, first I would like to thank you guys for this lib

I have a question about the Plug.Conn.req_headers. On plug documentation is written

* `req_headers` - the request headers as a list, example: `[{"content-type", "text/plain"}]`.

But on coap_ex, the req_headers keys are atoms instead of strings, there is a reason for this?

Example:

[
  content_format: "application/cbor",
  uri_host: "localhost",
  uri_path: ["broker", "api", "v2", "registry"],
  uri_port: 5683
]

Thanks in advance!