Body#split! fixed to not raise error in case missing boundary

When multipart email has no boundary specified in Content-Type header Body#split! raises ```TypeError```:

    can't convert NilClass to String

This behavior was introduced in PR #380. Prior to this parts were split like this:

    parts = raw_source.split("--#{boundary}")

In this case ```nil``` boundary is ok.
But after PR #380 it looks like this:

    parts = raw_source.split(/--#{Regexp.escape(boundary)}(?=(?:--)?\s*$)/)

Which, in case ```nil``` boundary causes ```TypeError```