Add the concept of a "default type". Normally the default type is

Add the concept of a "default type".  Normally the default type is
text/plain but the RFCs state that inside a multipart/digest, the
default type is message/rfc822.  To preserve idempotency, we need a
separate place to define the default type than the Content-Type:
header.

get_default_type(), set_default_type(): Accessor and mutator methods
for the default type.