Node 12 support - with suggested changes

[tomhog]

I've taken the this.response._headerNames part out of the catch statement

[k2s]

Call this.response._headerNames shows the same deprecation warning.
My fix proposal is:

    if (this.response.getHeaderNames) {
      this._headers = this.response.getHeaders() || {};
    } else {
      this._headers = this.response._headers || {};
      // Patch to node core
      this.response._headerNames = this.response._headerNames || {};
    }

[lamweili]

@darrendatbui has made the changes in PR #77 which superseded this PR.
Let's continue there and close this PR.

This has been superseded by PR #64.
This PR can be closed.

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