Deprecated use of _headers

[artursudnik]

When executed with nodejs v12, triggers deprecation warning: "[DEP0066] DeprecationWarning: OutgoingMessage.prototype._headers is deprecated"

The reason of it is here:

express-redis-cache/lib/ExpressRedisCache/route.js

Line 201 in df4ed8e

type: this._headers['content-type'],

[mayconmesquita]

Issue with more details: (#125)

I opened a pull request solving this issue (#126).