[BUG][Elixir] Wrong api client spec on multiple result types

Created by: bigardone

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When a reponse has different types, the generated client's spec ony refers to the type corresponding to a 200 as the result, when it should list all the possible types. Example:

Current generated version:

  @doc """
  Returns an user by `id`
  Returns an user account

  ## Parameters

  - connection (Connection): Connection to server
  - id (String.t): User ID
  - opts (KeywordList): [optional] Optional parameters
  ## Returns

  {:ok, %Model.UserShowSuccess{}} on success
  {:error, info} on failure
  """
  @spec fetch_user_by_id(Tesla.Env.client(), String.t(), keyword()) ::
          {:ok, Model.UserShowSuccess.t()} | {:error, Tesla.Env.t()}
  def fetch_user_by_id(connection, id, _opts \\ []) do
    %{}
    |> method(:get)
    |> url("/internal/users/#{id}")
    |> Enum.into([])
    |> (&request(connection, &1)).()
    |> evaluate_response([
      {200, %Model.UserShowSuccess{}},
      {404, %Model.NotFoundError{}},
      {500, %Model.InternalServerError{}}
    ])
  end

This, however, makes dialyzer throw warnings, because the @spec fetch_user_by_id shoud be:

 @spec fetch_user_by_id(Tesla.Env.client(), String.t(), keyword()) ::
          {:ok, Model.UserShowSuccess.t()} | {:ok, Model.NotFoundError.t()} | {:ok, Model.InternalServerError.t()} | {:error, Tesla.Env.t()}
  def fetch_user_by_id(connection, id, _opts \\ []) do
    # ...
  end

openapi-generator version

0.0.19-4.1.2

OpenAPI declaration file content or url

https://gist.github.com/bigardone/c656634c18d2e713c16de30537e013da

Command line used for generation

openapi-generator generate -i https://gist.githubusercontent.com/bigardone/c656634c18d2e713c16de30537e013da/raw/53d378b195de969712bf6db5d43154763449a8f7/OpenAPISpec.json -g elixir

Steps to reproduce

Related issues/PRs

Suggest a fix

All possible return types should be listed separated by | in here https://github.com/OpenAPITools/openapi-generator/blob/44e4dc3ff4765ae03ce5f1c91799d347b09ec498/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ElixirClientCodegen.java#L694