Response body comparator does not de-reference when reading content types

[jgoussard]

Issue

The response body comparator uses responses[status].content.* keys to find the existing content types. However, when the response is defined as a $ref, it needs to be de-referenced first.

Fix: #454

The issue was not detected in the tests because by default if no content type is found, application/json is used and it turned out to be the actual content type used in the fixture. The PR above ensures a test is added using text/plain content type.

[vwong]

PR accepted and released