Fix DateTime conversion issues with Int32
On 32-bit systems when using unix2datetime or julian2datetime and
passing a Int32 will result in a DateTime in an extremely limited range:
```
julia> Dates.unix2datetime(div(typemin(Int32), Int32(1000)))
1969-12-07T03:28:37
julia> Dates.unix2datetime(div(typemax(Int32), Int32(1000)))
1970-01-25T20:31:23
```
