implement/allow "required" attribute on response headers

[JasonWoof]

Please add support for the "required" attribute in header specifications.

The spec: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md#headerObject

says that header fields can contain (with exceptions) the same fields as parameters: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md#parameterObject

Which includes "required" which is supposed to default to false.

openapi-backend does not allow the "required" attribute and acts as though it were set to "true".

Changing to the default specified in the spec would break backwards compatibility, but if that's a problem you could at least allow an honor the "required" attribute when it exists.