Fix EmailHelper#newSession to return a distinct object at each call

Currently EmailHelper#newSession calls Session.getDefaultInstance which always return the same object after the initial instanciation

    public static synchronized Session getDefaultInstance(Properties props,
					Authenticator authenticator) {
	if (defaultSession == null)
	    defaultSession = new Session(props, authenticator);
...

Not sure if Session#getDefaultInstance should be used and prefered to a direct call to new Session(props, authenticator)

This change was introduced by NXP-9322 in Nuxeo 5.6