Fix dyalizer errors and warns

[gabrielgiordan]

After struggling with a Tesla.Mock.json/2 dialyzer warning at my application, I have found that the opts spec is a keyword list inside a list, instead of a plain keyword list.

So, I have analyzed other warnings and errors and also fixed those.

Checking PLT...
[:castore, :dialyxir, :elixir, :ex_doc, :exjsx, :fuse, :gun, :hackney, :ibrowse, :idna, :inets, :jason, :kernel, :logger, :mime, :mint, :mix_test_watch, :poison, :ssl, :ssl_verify_fun, :stdlib, :telemetry]
PLT is up to date!
No :ignore_warnings opt specified in mix.exs and default does not exist.

Starting Dialyzer
[
  check_plt: false,
  init_plt: '/Users/gabriel.medina/Git/tesla/_build/dev/dialyxir_erlang-22.3.4_elixir-1.10.3_deps-dev.plt',
  files: ['/Users/gabriel.medina/Git/tesla/_build/dev/lib/tesla/ebin/Elixir.Tesla.Middleware.JSON.beam',
   '/Users/gabriel.medina/Git/tesla/_build/dev/lib/tesla/ebin/Elixir.Tesla.Middleware.DecompressResponse.beam',
   '/Users/gabriel.medina/Git/tesla/_build/dev/lib/tesla/ebin/Elixir.Tesla.Middleware.Logger.beam',
   '/Users/gabriel.medina/Git/tesla/_build/dev/lib/tesla/ebin/Elixir.Tesla.Adapter.Gun.beam',
   '/Users/gabriel.medina/Git/tesla/_build/dev/lib/tesla/ebin/Elixir.Tesla.Middleware.Compression.beam',
   ...],
  warnings: [:unknown]
]
Total errors: 0, Skipped: 0, Unnecessary Skips: 0
done in 0m1.7s
done (passed successfully)

Edit: I suppose the CI is expected to fail at certificates_verification for test/tesla/adapter/mint_test.exs:59, as other PRs also failed at this step, is it right?

[teamon]

👋 Thank you for looking into that! I left a few comments.

cc @alex-strizhakov

[alex-strizhakov]

LGTM 👍