class Mail::Message

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

Returns the default value of the field requested as a symbol.

def default( sym, val = nil )
  if val
    header[sym] = val
  elsif field = header[sym]
    field.default
  end
end