kin-openapi openapi3filter: nil-pointer panic in ValidateResponse on a response header declared with content

Summary ValidateResponse panics on a nil pointer when a spec declares a response header with content instead of schema. doc.Validate accepts that document, so the crash comes from a spec the library just called valid. Same class as GHSA-jpcw-4wr7-c3vq, which covered the request-parameter case in...