OAS3: Accept header control

[shockey]

Fixes #3654.
CC #3641.

Adds ability to control Accept header.

Introduces the concept of a "blessed" response that dictates the Accept header value.

Logic is as follows:

if responses has a 2xx response code with media types defined
  use first 2xx response's media types as Accept options
else if responses has a default response with media types defined
  use default response's media types as Accept options
else
  use `*/*` as Accept value

Looks like: