It appears that Gnus currently will in certain cases incorrectly select
a MIME boundary that occurs within the message, resulting in a corrupt
message.  Specifically, it appears that `mml-boundary-function' is
called directly in a number of places outside of `mml-compute-boundary'
both in mml.el and in mml2015.el.  It seems that the correct behavior
is to use `mml-compute-boundary' in these cases.

The attached patch corrects these uses.