Base64 should be less strict with padding

This bug is submitted to re-look at java.util.Base64 to see it should be less strict when checking the padding, especially with MIME. Feedback from Bill Shannon is that there is a lot of incorrectly encoded mail and that he had to provide an option to JavaMail to control the strictness.

It has also been suggested that getEncoder(int,byte[]) should be renamed to getMimeEncoder.