unexpected behavior with Set?

Hi All,

Thanks for the great package!

I'm not sure if this behavior is expected, but it surprised me, so I thought I'd ask.

```
s = Set()
push!(s, 1us"day") # -> Set with 1 element
push!(s, 1us"day") # -> Set with 2 elements
```

I would have expected the set to only to have 1 element, because:
```
isequal(1us"day", 1us"day") # -> true
```

Am I mistaken in my intuition here?

Thanks for your time and work.
Cheers,
Max