[elixir] Decode list with primitives

Created by: mrmstn

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
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Description of the PR

In case a endpoint responds with a list of primitiv, the current elixir codegen tries to decode thouse to a struct which does not exist.

For example: The following should return a list of strings, this is what the codegen does (take a note at: %SwaggerPetstore.Model.String{}:

  @spec get_invoice(Tesla.Env.client, String.t, keyword()) :: {:ok, list(String.t)} | {:error, Tesla.Env.t}
  def get_invoice(connection, invoice_item_id, _opts \\ []) do
    %{}
    |> method(:get)
    |> url("/invoice/#{invoice_item_id}")
    |> Enum.into([])
    |> (&Connection.request(connection, &1)).()
    |> decode([%SwaggerPetstore.Model.String{}])
  end

This is what it does with this PR

  @spec get_invoice(Tesla.Env.client, String.t, keyword()) :: {:ok, list(String.t)} | {:error, Tesla.Env.t}
  def get_invoice(connection, invoice_item_id, _opts \\ []) do
    %{}
    |> method(:get)
    |> url("/invoice/#{invoice_item_id}")
    |> Enum.into([])
    |> (&Connection.request(connection, &1)).()
    |> decode([])
  end

To reproduce this, I made a small swagger sample - would be awesome if we coulde add this somehow to the samples

---
swagger: "2.0"
info:
  version: "1.0.0"
  title: "Swagger Petstore"
host: "petstore.swagger.io"
basePath: "/v2"
tags:
  - name: "invoice"
schemes:
  - "http"
paths:
  /invoice/{invoiceItemId}:
    get:
      tags:
        - "invoice"
      operationId: "getInvoice"
      parameters:
        - name: invoiceItemId
          in: path
          required: true
          type: string
      responses:
        200:
          description: successful operation
          schema:
            type: array
            items:
              type: string

cc: @wing328