Fix issue with isBehavior return true for arrays

[limemloh]

Purpose

Browsers are introducing an at function on arrays, which makes the isBehavior function return true for arrays.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at

Changes

• isBehavior now also checks for a "pull" property.

[paldepind]

What about only checking for pushB instead of both at and pull? pushB seems like a name that is fairly unlikely to collide with anything.

[limemloh]

What about only checking for pushB instead of both at and pull? pushB seems like a name that is fairly unlikely to collide with anything.

That sounds more reasonable, but it seems to give some issues
this future for example https://github.com/funkia/hareactive/blob/master/src/future.ts#L242

[stevekrouse]

Confirmed that this PR fixes the issue for all the browsers it was bugging on for me before