[elixir] Fixes `BadMapError` for header parameters

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.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

There's a issue with the current request_builder.ex since Tesla expects header parameters to be a map as they are unique. The request_builder.ex has in the current version no "special treatment" for the header parameters which will result in BadMapError since it tires to merge the map from tesla with the key-value list from the request_builder.

The full error message looks something like the following

iex(1)> SwaggerPetstore.Api.Invoice.get_invoice(SwaggerPetstore.Connection.new(), "123")
** (BadMapError) expected a map, got: [invoiceItemId: "123"]
    (stdlib) :maps.merge([invoiceItemId: "123"], %{"User-Agent" => "Elixir"})
    (tesla) lib/tesla/middleware/core.ex:107: anonymous fn/2 in Tesla.Middleware.Headers.merge/2
    (elixir) lib/map.ex:752: Map.update!/3
    (tesla) lib/tesla/middleware/core.ex:101: Tesla.Middleware.Headers.call/3
    (swagger_petstore) lib/swagger_petstore/api/invoice.ex:33: SwaggerPetstore.Api.Invoice.get_invoice/3

I made a small yml to reproduce the issue:

---
swagger: "2.0"
info:
  version: "1.0.0"
  title: "Swagger Petstore"
host: "petstore.swagger.io"
basePath: "/v2"
tags:
  - name: "invoice"
schemes:
  - "http"
paths:
  /invoice:
    get:
      tags:
        - "invoice"
      operationId: "getInvoice"
      parameters:
        - name: invoiceItemId
          in: header
          required: true
          type: string
      responses:
        204:
          description: successful operation

then I ran the follwing command in my iex (mix deps.get and iex -S mix) SwaggerPetstore.Api.Invoice.get_invoice(SwaggerPetstore.Connection.new(), "123")

Could you give me the honor and check my conclusion and code @wing328 ?