String to date conversions use current century

[kylemacey]

Previously, converting a String to Date with a 2 digit year would assume a year
between AD00 and AD99. As most software does not deal with these years, and it
is unlikely that this is considered expected behavior, String#to_date and
String#to_datetime now use the same defaults as Date#parse.

[rafaelfranca]

cc @pixeltrix

[claudiob]

I think this change makes sense, since it matches the default behavior of Ruby's stdlib.

@pixeltrix I can go on and rebase this commit against current master if you are 👍

[solojavier]

Any update on this? @claudiob @pixeltrix

[kylemacey]

I can rebase again (and remove my whitespace changes) if need be.

[pixeltrix]

I think I've said before that String#to_date and friends are not intended to be generic human text input parsers for a field in a form - there's not enough error checking for that and changing the behaviour for existing apps is also a reason not to merge this. There is now a ActiveSupport::TimeZone#strptime in Rails 5 that's available if you need to parse custom formats.

@kylemacey thanks for the PR, sorry I can't merge it.