Time#in_milliseconds preserves millisecond precision

`Time` stores the number of nanoseconds since the Epoch.
`Time#in_milliseconds` should return the number of milliseconds since the
Epoch with a millisecond accuracy.
Perhaps..

```ruby
  def test_in_milliseconds_with_duration
    assert_equal 10_000, 10.seconds.in_milliseconds
  end

  def test_in_milliseconds_with_numeric
    assert_equal 10_000, 10.in_milliseconds
    assert_equal 10_123, 10.123456.in_milliseconds
  end
```