Pairwise isn't tested for enumerator disposal

[atifaziz]

Pairwise tests don't test for enumerator disposal in:

MoreLINQ/MoreLinq.Test/PairwiseTest.cs

Lines 41 to 46 in ce57548

	[Test]
	public void PairwiseWideSourceSequence()
	{
	var result = new[] { "a", "b", "c", "d" }.Pairwise((x, y) => x + y);
	result.AssertSequenceEqual("ab", "bc", "cd");
	}

and:

MoreLINQ/MoreLinq.Test/PairwiseTest.cs

Lines 33 to 39 in ce57548

	public void PairwiseWithSequenceShorterThanTwo(int count)
	{
	var source = Enumerable.Range(0, count);
	var result = source.Pairwise(BreakingFunc.Of<int, int, int>());
	Assert.That(result, Is.Empty);
	}

This came up while reviewing issue #649. Thanks @Orace!