Optional of update messageId and sentDate

If where is no messageId, and sentDate when saveChanges() called, MimeMessage always generate new messageId and set sentDate if it is not exist.

This is confusing, as when changed only one header like "Message-ID" it will be rewritten on saveChanges() call. This can broke data consistency.

Is it possible save only updated values? Or make set those headers (Message-ID, Date) optionally updatable?

```java
protected synchronized void updateHeaders() throws MessagingException {
MimeBodyPart.updateHeaders(this);
setHeader("MIME-Version", "1.0");
if (getHeader("Date") == null)
setSentDate(new Date());
updateMessageID();

if (cachedContent != null) {
dh = new DataHandler(cachedContent, getContentType());
cachedContent = null;
content = null;
if (contentStream != null) {
try {
contentStream.close();
} catch (IOException ioex) { } // nothing to do
}
contentStream = null;
}
}
```