* src/engine/rfc822/rfc822-message.vala (Message): Fix has_plain_body(),
  handle the case where displayed MIME entities (as opposed to attached
  ones) with no Content-Type default to US-ASCII, per the RFC.

* test/engine/rfc822-message-test.vala (MessageTest): Add tests for
  testing and accessing body content as both plain text and HTML. Use
  GResources for accessing test message bodies rather than extremely long
  const strings.