Should `set` validate the resulting date/time value?

### Steps to reproduce

```elixir
iex(1)> {:ok, date} = Date.new(2018, 1, 31)
{:ok, ~D[2018-01-31]}
iex(2)> Timex.set(date, month: 2)
~D[2018-02-31]
```

### Description of issue

Expected result: `~D[2018-02-28]` or at least an error.