The prior commit focused on avoiding a panic when we encountered an invalid
boundary string. This commit makes thing degrade more gracefully: we'll
treat the bad MIME part in the same way that we deal with overly deep nesting
and handle it as an opqaque leaf part.
The new_multipart constructor now rejects an invalid boundary rather than
building an unserializable part, so a flawed policy can't accidentally create
broken messages.
Wez Furlong
b90d8bc1f3