fix(audit): use public APIs for accessing response headers
fix(audit): use public APIs for accessing response headers

- `res._headers` has been deprecated and accessing it in `auditResponseSerializer` generates exception (RangeError: Maximum call stack size exceeded)
- `res.getHeaders` returns an object that does not inherit from `Object`, so is unsuitable for use here
- falls back to `res._headers` if public APIs (introduced in v7.7.0) do not exist
- see https://nodejs.org/api/deprecations.html#deprecations_dep0066_outgoingmessage_headers_outgoingmessage_headernames
- see https://nodejs.org/api/http.html#http_response_getheaders

gergelyke/node-restify,myroid/node-restify,restify/node-restify,restify/node-restify,mcavage/node-restify,kusor/node-restify,hekike/node-restify,myroid/node-restify