title: openapi-generator for java is not able to generate class for oneOf field with primitive objects
body: <p>I have add the below property in my yaml file:</p>
  <pre class="lang-yaml prettyprint-override"><code>fieldValue:
      oneOf:
        - type: string
        - type: integer
  </code></pre>
  <p>Swagger-codegen version</p>
  <pre><code>       &lt;plugin&gt;
            &lt;groupId&gt;org.openapitools&lt;/groupId&gt;
            &lt;artifactId&gt;openapi-generator-maven-plugin&lt;/artifactId&gt;
            &lt;version&gt;4.1.1&lt;/version&gt;
         &lt;/plugin&gt;
  </code></pre>
  <p>CustomClass.java class is created with the field</p>
  <pre><code>public static final String SERIALIZED_NAME_FIELD_VALUE = &quot;fieldValue&quot;;
  @SerializedName(SERIALIZED_NAME_FIELD_VALUE)
  private OneOfstringinteger fieldValue = null;
  </code></pre>
  <p>But getting this error</p>
  <pre><code>/Users/....../CustomClass.java:[23,41] cannot find symbol
  [ERROR] symbol: class OneOfstringinteger
  </code></pre>
  <p>Is there a way to generate OneOfstringinteger class?</p>
  <p>I tried with objects also but same error came:</p>
  <pre class="lang-yaml prettyprint-override"><code>fieldValue:
      oneOf:
        - $ref: '#/definitions/StringValue'
        - $ref: '#/definitions/NumberValue'
  
  definitions:
    StringValue:
      type: object
      properties:
        name:
          type: string
  
    NumberValue:
      type: object
      properties:
        name:
          type: number
  
  </code></pre>
  <pre><code>cannot find symbol
  [ERROR]   symbol:   class OneOfStringValueNumberValue
  </code></pre>