fix: [DEP0066] DeprecationWarning: OutgoingMessage.prototype._headers is deprecated

[mythofleader]

Fixes: #130

tested node.js version: 8.17.0, 9.11.2, 10.18.0, 11.15.0, 12.13.0, 13.5.0

All files in test folder has been successfully passed except sails.js when node.js version >= 12.13.0

I found out that there are some places using res._headers in the file which path is node_modules/sails/lib/router/res.js.

I'm going to add this issue in sails framework.

[ajmedeio]

Thanks for getting this up, just ran into this today!

[arturnt]

@dougmoscrop any idea when this will get merged in?

[allexxis]

Ran into this problem today as well, I hope they merge this soon

[dougmoscrop]

Thanks for this. I will merge test and (soon) release