Using Dates.Time as dictionary keys

I found this problem when I tried using `Dates.Time` as dictionary keys:

```julia
julia> a = Dates.Time(0)
00:00:00

julia> b = Dates.Time(0) + Dates.Hour(24)
00:00:00

julia> a == b
true

julia> isequal(a, b)
true

julia> hash(a) == hash(b)
false
```