[BUG] [Java] Client resttemplate and webclient. Form Params are badly added when they are lists

[jorgerod]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Plugin will generate DefaultApi.java and when the element to be inserted to formParams is a list ... instead of an formParams.addAll it makes an formParams.add

openapi-generator version

4.1.3, 4.2.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: My Api
  version: 0.0.1
paths:
  /api/v1/article/id/exist:
    post:
      description: my endpoint
      operationId: checkArticle
      requestBody:
        required: true
        content:
          application/x-www-form-urlencoded:
            schema:
              type: object
              properties:
                articleIds:
                  description: article ids
                  type: array
                  items:
                    type: integer
                    format: int32
      responses:
        200:
          description: successful operation
          content:
            application/json:
              schema:
                type: integer

Command line used for generation

java -jar openapi-generator-cli.jar generate -g java -i openapi-rest.yml -o openapi --library resttemplate

Steps to reproduce

Run the above command.

Actual

    /**
     * 
     * my endpoint
     * <p><b>200</b> - successful operation
     * @param articleIds article ids (optional)
     * @return ResponseEntity&lt;Integer&gt;
     * @throws RestClientException if an error occurs while attempting to invoke the API
     */
    public ResponseEntity<Integer> checkArticleWithHttpInfo(List<Integer> articleIds) throws RestClientException {
     //...
        final MultiValueMap<String, Object> formParams = new LinkedMultiValueMap<String, Object>();

        if (articleIds != null)
            formParams.add("articleIds", articleIds);
     //...
   }

Expected:

    /**
     * 
     * my endpoint
     * <p><b>200</b> - successful operation
     * @param articleIds article ids (optional)
     * @return ResponseEntity&lt;Integer&gt;
     * @throws RestClientException if an error occurs while attempting to invoke the API
     */
    public ResponseEntity<Integer> checkArticleWithHttpInfo(List<Integer> articleIds) throws RestClientException {
     //...
        final MultiValueMap<String, Object> formParams = new LinkedMultiValueMap<String, Object>();

        if (articleIds != null)
            formParams.addAll("articleIds", articleIds);
     //...
   }

Related issues/PRs

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