Support object schemas in query items

[czechboy0]

We should support encoding non-primitive values as query items, which is prescribed by OpenAPI.

        - name: fields
          in: query
          description: Pairs of field=value to filter results
          required: true
          schema:
            type: object
            additionalProperties:
              type: string

The above example also uses additionalProperties, but I think that shouldn't change the story around serializing object structs into query items.

Somewhat related to #18.

This issue was also encountered in #83.