message: when parsing a message, don't treat absent header and empty header value the same

We now use "*string" for such header fields, for Content-* fields, as used in
the imapserver when responding to FETCH commands. We'll now return NIL for an
absent header, and "" (empty string) if the header value is empty.

imapserver/pack.go

@@ -34,11 +34,11 @@ func (t niltoken) xwriteTo(c *conn, xw io.Writer) {
 	xw.Write([]byte(t.pack(c)))
 }
 
-func nilOrString(s string) token {
-	if s == "" {
+func nilOrString(s *string) token {
+	if s == nil {
 		return nilt
 	}
-	return string0(s)
+	return string0(*s)
 }
 
 type string0 string