This fixes OAI#2349, which caught that an encoded PNG image
is encoded into a text media type.

In the process I realized some other errors, and simplified things.

* HTTP `Content-Type` is always handled by OAS
    * Media Type Object key in most cases
    * Encoding object (possibly inferred from schema) in `multipart/form-data`
* HTTP-level `Content-Encoding` is always handled by the OAS Header Object
* JSON Schema "content*" is used for embedding one media type into another
    * the encoded resource is of media type `text/plain`
    * `"contentMediaType"` is the embedded media type after decoding
    * `"contentEncoding"` is how to encode/decode binary to/from text

This removes any chance of `"contentMediaType"` conflicting with
the Media Type Object key or with `contentType` in the Encoding Object,
as they now always do different things.

Likewise, the HTTP `Content-Encoding` header (with values like
gzip, deflate, etc.) does different things than `"contentEncoding"`
(which has values like base64, base64url, quoted-printable, etc.).

The deprecated part header `Content-Transfer-Encoding` is likewise
handled in the Encoding Object, but is probably never used.

...rather than literal content format on the wire.

Fixed a typo and changed a SHOULD to MAY.

…tent-Encoding header

Co-authored-by: Mike Kistler <mkistler@us.ibm.com>