Assert.Throws conversion loses expected message

Given

```csharp
[Test, ExpectedException(typeof(ArgumentException), ExpectedMessage = "Must be bigger than zero")]
public void Foo()
{
    // method body
}
```

we convert it to

```csharp
[Xunit.Fact]
public void Foo()
{
    Assert.Throws<ArgumentException>(() =>
    {
        // method body
    });
}
```

so we lose the ExpectedMessage assertion