Fix round-tripping of empty media type objects and parsing of encoding style.

[darrelmiller]

Fixes for issue #683

1. Fix for parsing of encoding style.

2. Add support for both null and empty media type objects:

openapi: 3.0.1
info:
  title: empty vs null media type objects
  version: 1.0.0
paths:
  "/":
    get:
      responses:
        '200':
          description: ok
          content:
            text/plain: {}
            application/json: null