Messages 2.5 - Rethinking "9999" year in "birthdate" claim

[Michael Jones]

We currently use "9999" as the year in the "birthdate" claim to indicate that the year was not provided. Facebook and Google's other OAuth APIs use "0000" for this. I think we'll cause less developer confusion and bugs if we do the same.

[Nat Sakimura]

We changed from 0000 to 9999 because 0000 is a valid value, BC1.

See #597

[Michael Jones]

We will tentatively change back to 0000 to match existing practice. Vladimir's input is requested first, since he filed the original bug requesting us to use 9999.

[Nat Sakimura]

Duplicate of #597.