xUnit2013(Do not use Assert.Equal() to check collection size) should special-case ArraySegment<T>

[Gnbrkm41]

Similar issue (in fact, the root cause might be the same): #1206

Consider the following code:

        [Fact]
        public static void AsMemory_TryGetArray_ReturnsFalse()
        {
            ReadOnlyMemory<char> m = "0123456789".AsMemory();
            Assert.False(MemoryMarshal.TryGetArray(m, out ArraySegment<char> array));
            Assert.Null(array.Array);
            Assert.Equal(0, array.Offset);
            Assert.Equal(0, array.Count);
        }

The analyzer suggests that the last line should be changed to Assert.Empty(array.Count). However, because ArraySegment<T>.GetEnumerator() throws InvalidOperationException when the underlying array is null, the test fails with an uncaught InvalidOperationException.

[bradwilson]

Fixed in xunit/xunit.analyzers#133