Header parameter expects "required" even though its optional

Here it can be seen that the 'required' keyword is only required for path parameters: https://swagger.io/docs/specification/describing-parameters/ section "Required and Optional Parameters"

In the zds_client, the `required` keyword is also expected for header parameters: 
https://github.com/VNG-Realisatie/gemma-zds-client/blob/master/zds_client/schema.py#L100