code: def header=(header)
           if header.type == HeaderNode::UNIVERSAL
              @headers[0] = header
           elsif header.type == HeaderNode::LEFT_PAGE
              @headers[1] = header
           elsif header.type == HeaderNode::RIGHT_PAGE
              @headers[2] = header
           elsif header.type == HeaderNode::FIRST_PAGE
              @headers[3] = header
           end
        end
docs: This method assigns a new header to a document. A Document object can
   have up to four header - a default header, a header for left pages, a
   header for right pages and a header for the first page. The method
   checks the header type and stores it appropriately.
  
   ==== Parameters
   header::  A reference to the header object to be stored. Existing header
             objects are overwritten.