Cache references to Array.prototype.filter

[fatso83]

Purpose

Fixes issue #1521 by caching references to the filter method so that it can be stubbed. Also adds a hint to the pull request template on things that should be in place.

How to verify - mandatory

1. Check out this branch
2. npm install
3. npm test

[fatso83]

I added no extra tests for the second commit. I simply couldn't find out how to add relevant tests, so I gave up after close to an hour of messing around with call.js and call-test.js, but at least all existing tests pass.

[mantoni]

LGTM.

[mroderick]

This has become part of v3.2.0