fix: OnlyIf for chained constraints on typed exceptions

[vbreuss]

Building on top of #35 fixes #34 also for typed exceptions:

  [Fact]
  public async Task ShouldSupportChainedConstraints()
  {
    Action action = () => { };
  
    await Expect.That(action).Throws<MyCustomException>()
      .OnlyIf(false)
      .Which.HasMessage("foo");
  }