[BUG] [Java Resttemplate] Incorrect handling of free form query parameters

Created by: gspatace

Description: Currently, when using free form query parameters, the URI is not built correctly.

Steps to reproduce: Input spec file:

openapi: 3.0.0
info:
  title: test
  version: 0.0.1
servers: 
  - url: "http://localhost"
paths:
  /some/endpoint: 
    get: 
      parameters: 
        - in: "query"
          name: "fixed"
          schema: 
            type: "string"
        - in: "query"
          name: "free-form"
          schema:
            type: "object"
          style: "form"
      responses: 
        200:
          description: "test"

Command to generate client files: java -jar modules\openapi-generator-cli\target\openapi-generator-cli.jar generate -i spec.yml -o work -g java --library resttemplate

Client code ( usage of generated API ):

        ApiClient apiClient = new ApiClient();
        DefaultApi defaultApi = new DefaultApi(apiClient);
        Map<String, Object> freeFormQueryParams = new HashMap<>();
        freeFormQueryParams.put("this", "first");
        freeFormQueryParams.put("that", "second");
        freeFormQueryParams.put("other", "third");
        defaultApi.someEndpointGet("fixedValue", freeFormQueryParams);

Expected result of URI built for the endpoint: http://localhost/some/endpoint?fixed=fixedValue&this=first&that=second&other=third

Actual result: http://localhost/some/endpoint?fixed=fixedValue&free-form=%7Bthat%3Dsecond%2C+other%3Dthird%2C+this%3Dfirst%7D

Tested with master as of today.

I've added a patch that addresses this, I'll open a PR soon, too.

free-form-params.txt