fix(Entity): Do not add Array.prototype properties to store

[livthomas]

I have tried to fix issue #781 and verified the problem is really no longer there by building my application (where I can always reproduce the problem) with a version of @ngrx/entity containing these changes.

[coveralls]

Coverage increased (+0.2%) to 93.016% when pulling f311f39 on livthomas:entity-null into ecf1ebf on ngrx:master.

[MikeRyanDev]

It sounds like this is hard to reproduce. Any chance you can isolate it into a unit test that fails on current version but passes with this fix?

[livthomas]

I simply cannot reproduce it in the unit tests. But I think I have found the root cause. My project uses core-js library which adds some additional Array.prototype methods and the for..in loop in @ngrx/entity code then iterates over them (as well as over the indices).

In general, iterating over an array with for..in loop is a very bad idea and should be avoided. See the relevant sections of Speaking JavaScript book or MDN documentation.

[livthomas]

@MikeRyanDev, I have added a reproducer here as you suggested.