Accept header with multiple types default to "text/plain"

[Mcourt]

Describe the bug
When I request prism with an "accept" header that has multiple types (ex: application/json, text/plain, */*), prism will try to send back "text/plain" response.

To Reproduce

1. Given this OpenAPI document

openapi: 3.0.0
info:
  title: My title
  version: 1.0.0
paths:
  /check-email:
    post:
      tags:
        - User
      summary: Check the availability of an email
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                email:
                  type: string
                  example: [email protected]
        required: true
      responses:
        "204":
          description: OK, the email is available
          content:
            application/json:
              schema:
                nullable: true
        "400":
          description: Bad Request, the email is not available
          content:
            application/json:
              schema:
                nullable: true

2. Run this CLI command http POST 127.0.0.1:4010/check-email [email protected] Accept:'application/json, text/plain, */*'
3. See error

{
    "detail": "Cannot serialise complex objects as text/plain",
    "status": 500,
    "title": "Cannot serialise complex objects as text/plain",
    "type": "https://stoplight.io/prism/errors#NO_COMPLEX_OBJECT_TEXT_PLAIN"
}

Expected behavior
I expected the server to respond with a 204 response.

Environment (remove any that are not applicable):

• Library version: 3.1.0
• OS: Ubuntu 18.04.3 LTS (Bionic Beaver)

Additional context
I am using "axios" to make the call and it defaults to application/json, text/plain, */*' for its 'Accept' header.