Refactor response handling

[danschultzer]

I'm not happy with how responses are currently handled. As an example here's how an error response is handled:

https://github.com/danschultzer/ex_oauth2_provider/blob/9a8fc6dbd2527ac06a56e97b97411caa47ea87f7/lib/ex_oauth2_provider/oauth2/authorization.ex#L27-L34

https://github.com/danschultzer/ex_oauth2_provider/blob/9a8fc6dbd2527ac06a56e97b97411caa47ea87f7/lib/ex_oauth2_provider/oauth2/authorization.ex#L48-L49

https://github.com/danschultzer/ex_oauth2_provider/blob/9a8fc6dbd2527ac06a56e97b97411caa47ea87f7/lib/ex_oauth2_provider/oauth2/authorization.ex#L54-L59

https://github.com/danschultzer/ex_oauth2_provider/blob/9a8fc6dbd2527ac06a56e97b97411caa47ea87f7/lib/ex_oauth2_provider/oauth2/utils/error.ex#L5-L9

https://github.com/danschultzer/ex_oauth2_provider/blob/9a8fc6dbd2527ac06a56e97b97411caa47ea87f7/lib/ex_oauth2_provider/oauth2/authorization/utils/response.ex#L13-L14

https://github.com/danschultzer/ex_oauth2_provider/blob/9a8fc6dbd2527ac06a56e97b97411caa47ea87f7/lib/ex_oauth2_provider/oauth2/authorization/utils/response.ex#L31-L38

From the above it's almost impossible to figure what's going on. The code is fragmented and you have to go back and fourth between several files to figure out what's going on. Also the response doesn't make much sense since it's a tuple with a map where an :error key value has been added. Not very helpful.

It should be a lot more compact an easy to read. Something along these lines, almost everything contained within this module:

  def authorize(resource_owner, request, config \\ []) do
    request
    |> validate_response_type(config)
    |> case do
      {:error, error}     -> response_type_validation_error({:error, error}, request, config)
      {:ok, token_module} -> token_authorize(token_module, resource_owner, request, config)
    end
  end

  defp response_type_validation_error({:error, error}, request, config) do
    # Return HTTP error code or build redirect response
  end

  defp token_authorize(Code, resource_owner, request, config), do: Code.authorize(resource_owner, request, config)