lib/date.rb:  Backport #2707 [ruby-core:28011]; backport r27014 to fix problem with Date#>> and very small numbers.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8_6@28233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e