def default( sym, val = nil )
      if val
        header[sym] = val
      elsif field = header[sym]
        field.default
      end
    end
def default( sym, val = nil )
      if val
        header[sym] = val
      elsif field = header[sym]
        field.default
      end
    end
Returns the default value of the field requested as a symbol.

 Each header field has a :default method which returns the most common use case for
 that field, for example, the date field types will return a DateTime object when
 sent :default, the subject, or unstructured fields will return a decoded string of
 their value, the address field types will return a single addr_spec or an array of
 addr_specs if there is more than one.