Fix assertions incorrectly failing when a tolerance of "zero" for the type is used.

I migrated Polly from FluentAssertions to Shouldly over the weekend and we had a test like this:

value.Should().BeCloseTo(expected, TimeSpan.Zero); // value is a TimeSpan

Migrating to Shouldly, I changed this to:

value.ShouldBe(expected, TimeSpan.Zero);

The test then failed despite the values being equal.

I guess the original author wanted to be explicit? This was easily resolved by just changing the assertion to value.ShouldBe(expected).

This PR fixes that behaviour by changing < to <= for TimeSpan, as well as decimal, double, DateTime and DateTimeOffset that also have the same issue.

If non-zero tolerances being equal shouldn't match, then an alternative fix could be to special-case "zero" for these cases in Is.