Fix Date.range/3 raises wrong exception for invalid step

[AlexGx]

Fixes Date.range/3 raises wrong exception for non-printable (does not implements String.Chars protocol) step

Current

iex> Date.range(~D[2001-01-01], ~D[2002-01-01], {1})
** (Protocol.UndefinedError) protocol String.Chars not implemented for Tuple

Got value:

    {1}
...

Expected

** (ArgumentError) both dates must have matching calendar and the step must be a non-zero integer, got: ~D[2001-01-01], ~D[2002-01-01], {1}

[josevalim]

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