Core Extension String#to_datetime ignores time zone

When calling to_datetime on a string like "2009-05-15T09:51:33+02:00", the time zone is currently ignored.

>> "2009-05-15T09:51:33+02:00".to_datetime.xmlschema
=> "2009-05-15T09:51:33+00:00"

This breaks, as of Ruby 1.8, the standard lib in lib/ruby/1.8/xsd/datatypes.rb:

561  def screen_data(t)
562    # convert t to a DateTime as an internal representation.
563    if t.respond_to?(:to_datetime)      # 1.9 or later
564      t.to_datetime
565    elsif t.is_a?(DateTime)
566      t

Patch with tests is attached.

Kind regards
Christoph Petschnig