get_content_type(), get_content_maintype(), get_content_subtype(): RFC

get_content_type(), get_content_maintype(), get_content_subtype(): RFC
2045, section 5.2 states that if the Content-Type: header is
syntactically invalid, the default type should be text/plain.
Implement minimal sanity checking of the header -- it must have
exactly one slash in it.  This closes SF patch #597593 by Skip, but in
a different way.

Note that these methods used to raise ValueError for invalid ctypes,
but now they won't.