[DEP0066] Replaced deprecated _headers property with getHeaders()

[gnurag]

expressjs apps using express-redis-cache log this error on the console:

(node:57103) [DEP0066] DeprecationWarning: OutgoingMessage.prototype._headers is deprecated
(Use `node --trace-deprecation ...` to show where the warning was created)

Replacing the deprecated _headers property with getHeaders() fixes this.

ref: DEP0066 - https://nodejs.org/api/deprecations.html#DEP0066