TimeWithZone should not accept unrecognized time zones

Imported from Lighthouse. Original ticket at: http://rails.lighthouseapp.com/projects/8994/tickets/6573 Created by Marc-André Lafortune - 2011-03-14 14:53:36 UTC

Currently, the following doesn't raise any error:

t = Time.now.in_time_zone("There is no such timezone"); t.utc

Many other methods of TimeWithZone will raise a NoMethodError as soon as they try to access the timezone (e.g. t.inspect).

It should not be possible to create a TimeWithZone with an invalid timezone.