It should be possible to set real data for bodies

[SvenBayer]

A request- and response-body should be able to have example data in the x-example field as json string so we can create requests and responses with real data.

Example of a response with real data:
responses:
'201':
description: Created
schema:
x-example: "{ id: 1, name: 'Mark Muller' }"
$ref: '#/definitions/BeanPlanet'

Acceptance criteria

• It is verified or implemented that a x-example field can be set to a json string
• The spring-cloud-contract-swagger-sample project is extended
• Tests are added