Extract body from nested multiparts

The camel-mail component currently handle mails with attachments where the mail body is a plain text message. In this case the MIME-Structure looks as follows:

mulipart/mixed
|- text/plain   -> body
|- image/png    -> attachment[0]
|- text/plain   -> attachment[1]

However if the mail is e.g. sent by Outlook and the mail body contains formating and a picture, the resulting multipart structure might look like this

multipart/mixed
|- multipart/related
|  |- multipart/alternative
|  |  |- text/plain  -> body
|  |  |- text/html
|  |- image/png      -> attachment[0]
|- text/plain        -> attachment[1]

Without the patch, the attachments will be as expected, but the body will be the same as attachment[1] (or some bogus string if the mail does not contain a text attachment).