ApiResponse annotation missing headers

[arthurdm]

According to the spec, Response should have a headers field: https://github.com/OAI/OpenAPI-Specification/blob/OpenAPI.next/versions/3.0.md#responseObject

This maps to the ApiResponse annotation.

[janamanoharan]

https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#responseObject