Cast datetime to str before setting 'date' key in RedisTarget

[KenyaDonDraper]

Fixes #2721

[dlstadther]

@Gr1f0n6x , given you added Redis support, could you confirm this shouldn't negatively impact existing uses of this module?

[nryanov]

@dlstadther , unfortunately, I'm not an author of this module. The real author is @pbamba (#2489).
But, as I think, this change should not has negative impact. For existing code it will be equal to double call to str. It may add some overhead (probably, too small for worrying).