Spanner.date generates an invalid Spanner DATE value for dates before 1000AD

[viralpickaxe]

Spanner.date does not generate Spanner compatible ISO dates for years before 1000AD, for example:
Spanner.date("954-01-01").toJSON() => 954-01-02 (instead of 0954-01-02)

This causes an INVALID_ARGUMENT query error

[asthamohta]

Hi @viralpickaxe, thanks for bringing this to our attention. I feel this code might be a breaking change for someone who is expecting 954-01-02 and has designed their code that way so what I can do is add another new function toIsoStandardJSON() with your code. Would that be okay?

[viralpickaxe]

Hi @asthamohta, this seems like a pretty bad duct tape solution to a bug that is obviously a mistake rather than intended behaviour, see below the official node.js and java spanner SDK docs:

[NodeJS] SpannerDate - toJSON()

Returns the date in ISO date format. YYYY-[M]M-[D]D

[Java] Struct SpannerDate (4.0.0) - ToString()

Returns the ISO 8601 representation (yyyy-MM-dd) of the date represented by this instance.

Sure this may be a breaking change, but adding a new confusing second serialiser (that javascript doesn't use by default) seems like the wrong answer here, instead of bumping a version

[asthamohta]

Okay, that sounds fair @viralpickaxe. I'll merge your PR :)