Align this handing with ES5 on array builtins

[bjouhier]

Array builtins are currently inconsistent with ES5 builtins in the way they handle this. There are two differences:

• If thisArg is omitted, the callback is called with this set to undefined in ES5 (forEach, map, ...) but to the array itself in streamline's builtins (forEach_, map_, ...).
• reduce_ and reduceRight_ take a thisArg parameter; reduce and reduceRight don't!

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach#Parameters and related pages.

I propose to fix this and align streamline's builtins on the ES5 array builtins.

[bjouhier]

Travis-ci is happy now :smile:

[bjouhier]

Issue is still pending, though. I've only disabled the unit tests that were revealing the issue.