JSON Schema: Support default values and array items type

Thanks to @jgazeau and @TiboStev for contributing the following changes:

• Support default values for ParameterDescriptors in JSON Schema generated by OpenAPI 2 and OpenAPI 3 generators, see #187 , #188
• Support specifying the array items type via attributes of a FieldDescriptor (e.g. https://github.com/ePages-de/restdocs-api-spec/pull/186/files#diff-1692be3c3118e65fd0f7e339973764c4af3a504aae887fad431cda58b876cbfeR408 ). This renders as items.type for the respective JSON Schema. Fixes #147 and part of #146 (other part is untested), full change: #186