Avoid deprecation warning: use this.getHeaders from node v12+

[mayconmesquita]

Issue #125

Node version 12 deprecated OutgoingMessage.prototype._headers, which is used to create cache route.

The line using this._headers: ./node_modules/express-redis-cache/lib/ExpressRedisCache/route.js:200:30

Instead of using this._headers, we need to use obj.getHeaders() to get the headers object.

So we should replace the property name ._headers with the function .getHeaders(). Same results using either one except using the function is the supported method and removes the deprecation warning.

Pull Request

I used try/catch to make this compatible with all node versions.

try {
  contentType = this.getHeaders()['content-type']; // method added in node v7.7.0
} catch (err) {
  contentType = this._headers['content-type']; // required by pre v7.7.0 node [deprecated since v12.0.0]
}