[BUG] [SPRING] Incorrect initialization class of model's property

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

While generating Model with Map/Dictionary Properties with the help of 'additionalProperties' i receive

public class Error400AIS   {
  @JsonProperty("_links")
  private LinksAll links = new HashMap<>();

but expecting

public class Error400AIS   {
  @JsonProperty("_links")
  private LinksAll links = new LinksAll();

openapi-generator version

4.3.1

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: NextGenPSD2 XS2A Framework
  version: "1.3.4_2019-07-17v1"

paths:
  /v2/ping:
    get:
      summary: Ping service
      description: Ping-pong
      responses:
        '200':
          description: OK
          content:
            text/plain:
              schema:
                type: string

components:
  schemas:
    hrefType:
      description: Link to a resource
      type: object
      properties:
        href:
          type: string
    _linksAll:
      description: |
        A _link object with all availabel link types
      type: object
      additionalProperties: 
        $ref: "#/components/schemas/hrefType"
      properties:
        last:
          $ref: "#/components/schemas/hrefType"
    Error400_AIS:
      description: Standardised definition of reporting error information according to [RFC7807]
      type: object
      properties:
        _links:
          $ref: "#/components/schemas/_linksAll"

Command line used for generation

openApiGenerate {
    verbose.set(false)
    generatorName.set("spring")
    validateSpec.set(false)
    inputSpec.set("$rootDir/specs/psd2-api 1.3.4 20190717v1_short.yaml")
    configFile.set("$rootDir/specs/config.json")
    outputDir.set("$projectDir/src/main/java-generated")
    additionalProperties.set(mapOf("sourceFolder" to ""))
    systemProperties.set(mapOf("modelDocs" to "false", "models" to ""))
    supportingFilesConstrainedTo.set(emptyList())
}

{
  "dateLibrary": "java8",
  "hideGenerationTimestamp": true,

  "apiPackage": "psd2.api",
  "modelPackage": "psd2.dto",
  "configPackage": "psd2.conf"
}

Steps to reproduce

Just try to create model with 4.3.1 generator

Related issues/PRs

Could not find

Suggest a fix

Probably fix should goes "modules/openapi-generator/src/main/resources/JavaSpring/pojo.mustache" 40th line. Or "defaultValue" should be equals to "datatypeWithEnum" (but have no idea what could be broken in this case)

Start with modules/openapi-generator/src/main/resources/JavaSpring/pojo.mustache around line 40 and reproduce the model using the OpenAPI declaration in the issue. Compare the generated initialization for the _links property with the declared _linksAll type and verify that the output uses LinksAll rather than HashMap.