fix: DateRange invalid slice step computation when step != 1

[AlexGx]

Current behavior

Enum.slice(Date.range(~D[2001-01-01], ~D[2001-01-10], 2), 0..2//2)
#=> [~D[2001-01-01], ~D[2001-01-04]]

~D[2001-01-04] - is not at range at all

Expected

Enum.slice(Date.range(~D[2001-01-01], ~D[2001-01-10], 2), 0..2//2)
#=> [~D[2001-01-01], ~D[2001-01-05]]

Changes

• fix slice step computation
• tests updated

[josevalim]

💚 💙 💜 💛 ❤️