Change the way SpannerDate returns value.

[ghost]

Fixes #526 (it's a good idea to open an issue first for discussion)

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[callmehiphop]

@sulaysumaria so interestingly enough this change will cause an off-by-one error since the Date constructor interprets Y-m-d timestamps UTC-0 and those date methods all return local time values. I think the underlying issue here is that the Date object might not be the wrong way to go since it infers a timezone based on the format of the date string. I'm going to hold off on this PR for now and try to open a dialogue on how to move forward with this bug.