OAS3 fails to load a resource when encoding is missing 'style'

[chris-miaskowski]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce

openapi: '3.0.1'
paths:
  /path:
    post:
      responses:
        200:
          content:
            text/plain:
              example: ok
      requestBody:
        content:
          application/x-www-form-urlencoded:
            schema:
              type: object
              properties:
                reserved:
                  type: string
            encoding:
              reserved:
                contentType: text/plain

prism mock file.oas3.yaml 

Expected behavior

Load the file and create server with one endpoint

Actual

✖  fatal     Encoding property style: 'undefined' is incorrect, must be one of: form,spaceDelimited,pipeDelimited,deepObject

Environment (remove any that are not applicable):

• Library version: 3.0.3