Add diff and intersect to Support\Collection and merge, diff and intersect to Eloquent\Collection

[dwightwatson]

The diff and intersect functions on Illuminate\Support\Collection use native PHP functions while merge, diff and intersect functions on `Illuminate\Eloquent\Collection' work by using the primary key of provided models.

Tests included.

[taylorotwell]

Why does it not use the built in PHP array diff, intersect, stuff?

[dwightwatson]

The built-in functions look for identical objects where the same model may not always be so, so these use the primary key of the models to determine uniqueness.

[taylorotwell]

OK my personal thinking for how this should wrok would be to name them just "diff", 'Intersect", etc. and move a base version of them down intos upport collection, which just uses the PHP native functions, then override those functions in Eloquent with what you ahve here for key checking.

[dwightwatson]

Alright, I'll implement that in the pull request. Thanks!

[JoostK]

This code is now duplicated over three methods, perhaps it could be extracted?

[dwightwatson]

I considered this. Maybe a protected getArrayableItems method on Illuminate\Support\Collection?

[JoostK]

Sounds good to me.