#243654: only create a new MIME boundary if we don't already have one.
#243654: only create a new MIME boundary if we don't already have one.

The rearranged code should do exactly what the old code did, but
the new code avoids a potentially costly re computation in the case
where a boundary already exists.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator