Support objects created with Object.create(null)

[asprouse]

Summary

Proposed change:

Support objects created with Object.create(null) by following the no-prototype-builtins. This fix is important because it makes nunjucks compatible with this widely used ES5 feature (Object.create).

Closes #468

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

Note no documentation update was necessary.

[ArmorDarks]

This isn't related to PR directly, but I still can't get why would someone create object from null?

[ArmorDarks]

Also, here #468 (comment) said that this method doesn't work that well in older browsers.

[asprouse]

@ArmorDarks creating an object using Object.create(null) creates an object without a prototype which is suitable for using as a map. Instead of calling obj.hasOwnProperty(x) to check membership one can simply obj[x]. It seems that this is a technique used is several popular libraries (express and graphql off the top of my head).

As far as compatibility goes this PR only changes how hasOwnProperty is called obj.hasOwnProperty(x) -> Object.prototype.hasOwnProperty.call(obj, x). Both hasOwnProperty and call predate ES5 (ES3 and ES1 according to MDN) so even IE8 support shouldn't be a problem.

[fdintino]

Thanks for the PR! I agree that Object.prototype.hasOwnProperty.call(obj) is preferable to obj.hasOwnProperty().