Add basic parsing of RFC 2017 external bodies

* At least parses the [RFC 2017](http://tools.ietf.org/html/rfc2017) `external-body` content type rather than aborting.
* Fixture is from [dev@httpd, publicly available archive](http://mail-archives.apache.org/mod_mbox/httpd-dev/199712.mbox/%3CPine.BSF.3.95.971201104619.22475B-120000@alive.znep.com%3E).