Make Time#to_date compliant with MRI 1.9-2

Time#to_date should use the proleptic Gregorian calendar to construct a Date,
but should return a Date object with the default calendar reform day. This is
unlike the behavior of Ruby 1.8 Time#to_date, which was private and seems to
have served as the basis for the current implementation. A specification
for this method has been added to rubyspec.