Ignore false Content-Type headers in multipart epilogues

Here is an modified mime from the real world:

```
Content-Type: multipart/mixed;
	boundary="_001_538f5bb0a7956_457a8046c758764ef_"
Mime-Version: 1.0
Date: Mon, 04 Jun 2014 13:50:00 -0400
Subject: test message via curl - extra content-type (_001_)


--_001_538f5bb0a7956_457a8046c758764ef_
Content-Type: multipart/alternative;
	boundary="_000_538f5bb0a7956_457a8046c758764ef_"

This is the preamble.  It is to be ignored, though it
is a handy place for mail composers to include an
explanatory note to non-MIME compliant readers.
--_000_538f5bb0a7956_457a8046c758764ef_
Content-Type: text/plain

Test - body content

--_000_538f5bb0a7956_457a8046c758764ef_--
This is the epilogue.  It is also to be ignored.
Content-Type: text/html

--_001_538f5bb0a7956_457a8046c758764ef_--
```

The bug was that `Content-Type: text/html` was recognized by tokenizer, when in fact it was just a text that should have been ignored.
Why have you decided to use `isinstance` instead of an explicit method?