code: def restriction!(restriction)
        @nbuild[NS_EWS_MESSAGES].Restriction {
          restriction.each_pair do |k,v|
            self.send normalize_type(k), v
          end
        }
      end
docs: Build the Restriction element
   @see http://msdn.microsoft.com/en-us/library/aa563791.aspx
   @param [Hash] restriction a well-formatted Hash that can be fed to #build_xml!