Doesn't honor content-type parameter's default setting

[glinton]

For post requests that have no payload, but content-type is required, a requestBody definition is required so that oats will populate that request header.

This swagger produces the following ts:

openapi: 3.0.0
info:
  title: Test API
  version: 0.0.0
servers:
  - url: ''
paths:
  /a:
    post:
      parameters:
        - in: header
          name: Content-Type
          required: true
          schema:
            type: string
            default: 'text/plain; charset=utf-8'
      responses:
        '200':
          description: Success
  /b:
    post:
      requestBody:
        required: true
        content:
          text/plain; charset=utf-8:
            schema:
              type: object
      responses:
        '200':
          description: Success

export interface PostAParams {
  headers: {
    "Content-Type": string;
  };
}

export const postA = (
  params: PostAParams,
  options: RequestOptions = {}
): Promise<PostAResult> =>
  request("POST", "/a", params, options) as Promise<PostAResult>;

export const postB = (
  params: PostBParams,
  options: RequestOptions = {}
): Promise<PostBResult> =>
  request(
    "POST",
    "/b",
    { ...params, headers: { "Content-Type": "text/plain; charset=utf-8" } },
    options
  ) as Promise<PostBResult>;