Header parameters are not generated according to openapi spec

[prasannakumar1989]

Thank you for this amazing gem!

The schema option need to be added for header parameters. According to open api 3.0.0, parameters should have a schema property.

...
paths:
  /post:
    post:
      parameters:
        - in: header
          name: X-username
          schema:
            type: string
            example: user12345

Currently headers don't have schema property.

paths:
  /post:
    post:
      parameters:
        - in: header
          name: X-username
          example: user12345

Can you please address this issue ?