handle examples with nested properties pagination

[w00kie]

Referring to #1236, I've made the build_listed_example_value function travel recursively down the schema properties to generate a paginate example.

This should enable examples to work in case of custom pagination classes that have nested properties, such as one like this:

{
  "pagination": {
    "limit": 100,
    "offset": 300,
    "total": 123,
    "next": "/accounts/?offset=400&limit=100",
    "previous": "/accounts/?offset=200&limit=100"
  },
  "data": [
    {"field": 123}
  ]
}

[tfranzel]

good catch! thanks for the excellent PR!