mails from iDempiere can not be forwarded

Tested and it's ok in my case (alerts opened on IOS, see https://groups.google.com/forum/?fromgroups=#!topic/idempiere/Poz0Hq7Kw2Q)

Regards,

Nicolas

fix error NPE report by @Nicolas Micoud

@Carlos Ruiz, @Thomas Bayen please help me review my patch.
i test it's ok for this issue and IDEMPIERE-1899. but has below limit:

current when make email, just html content is add,
with old code (user multipart/related), in text mode, thunder-bird can get html content as plan text.

but with new code, thunder-bird in text mode can't display content without improve code.
in case this patch is test ok and apply, we can pass limit by use a library (http://jsoup.org/) to get text plan from html and add more to email conten.

Backed out the change to mixed - it broke https://idempiere.atlassian.net/browse/IDEMPIERE-1899#icft=IDEMPIERE-1899

Yes, I think this is the same issue. As I analyzed the similar issues I could found via google some thunderbird users have this problem and some not. It seems to depend not only on a special thunderbird version but on the mail program that the sender uses. In my thunderbird I receive many mails and checked a lot of them today. Most mails can be forwarded without problems. Some can not. iDempiere mails can not. I looked what was the difference and found out that iDempiere uses "related" as the multipart mimetype. After reading the wikipedia entry about mimetypes I decided that "mixed" is the better choice. After changing iDempiere to use "mixed" the problem was gone.

As a conclusion I think that iDempiere uses the multipart mimetype in a wrong way. This leads to a special behaviour of Thunderbird that is annoying for me. Other MUAs can go over this wrong mimetype and behave better - I did not check. But In my opinion "mixed" should work everywhere.