Fixes Dialyzer error for Guardian.Plug.Pipeline.implementation/0

[holsee]

Addresses issue whereby dialyzer will warn that "Type specification is a supertype of the success typing." on the generated implementation/0 function when using Guardian.Plug.Pipeline.

This is technically an "underspecification" error. See: http://erlang.org/doc/man/dialyzer.html -Wunderspecs

Full Dialyzer Warning:

lib/my_app/authorisation/guardian.ex:2:contract_supertype
Type specification is a supertype of the success typing.

Function:
MyApp.Authorisation.Guardian.Plug.implementation/0

Type specification:
@spec implementation() :: module()

Success typing:
@spec implementation() :: MyApp.Authorisation.Guardian

Usage:

defmodule MyApp.Authorisation.Pipeline do
  use Guardian.Plug.Pipeline,
    otp_app: :my_app,
    error_handler: MyApp.Authorisation.ErrorHandler,
    module: MyApp.Authorisation.Guardian  # <- where implementation is passed, that is supertype of `module()`

  plug Guardian.Plug.VerifyHeader, realm: "Bearer"
  plug Guardian.Plug.EnsureAuthenticated
  plug Guardian.Plug.LoadResource
end

I feel this is a minor issue, but given the fact this code is generated for the specific "module" the direct use of the unquoted module name (in this case MyApp.Authorisation.Guardian) should be fine.

[codecov-io]

Codecov Report

Merging #572 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #572   +/-   ##
=======================================
  Coverage   85.57%   85.57%           
=======================================
  Files          18       18           
  Lines         423      423           
=======================================
  Hits          362      362           
  Misses         61       61

Impacted Files	Coverage Δ
lib/guardian/plug.ex	83.78% <ø> (ø)	:arrow_up:

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