BitArray throws IndexOutOfRangeException when unmet shrink threshold

[bricelam]

var left = new BitArray(64);
var right = new BitArray(32);

// NB: Since the shrink threshold (256) isn't met, the underlying int array isn't resized
left.Length = 32;

// System.IndexOutOfRangeException: Index was outside the bounds of the array.
left.Or(right);

The Or operation (and probably others) assume that the length of underlying int arrays of both operands are equal. But this isn't the case when the arrays aren't actually resized when the BitArray length changes.

cc @ajcvickers @Pilchie

[Pilchie]

@danmosemsft ?

[danmoseley]

@tannergooding maybe one you could look at?

[Gnbrkm41]

I think I can submit a PR for this. Feel free to reject if it's undesirable 😁