[BUG] [Java] Escaped bean property name should be resumed

Description

xref: https://github.com/kubernetes-client/java/pull/595

in the kubernetes openapi spec, there's some escaped property name in the schema:

    "io.k8s.apiextensions-apiserver.pkg.apis.apiextensions.v1beta1.JSONSchemaProps": {
      "description": "JSONSchemaProps is a JSON-Schema following Specification Draft 4 (http://json-schema.org/).",
      "properties": {
        "$ref": {
          "type": "string"
        },
        "$schema": {
          "type": "string"
        }
....

the generator doesn't seem to properly handle the name and my java output is failing the compiliation:

@ApiModel(description = "JSONSchemaProps is a JSON-Schema following Specification Draft 4 (http://json-schema.org/).")
@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", date = "2019-06-20T12:55:05.588Z[Etc/UTC]")
public class V1beta1JSONSchemaProps {
  public static final String SERIALIZED_NAME_$_REF = "$ref";
  @SerializedName(SERIALIZED_NAME_$_REF)
  private String $ref;

  public static final String SERIALIZED_NAME_$_SCHEMA = "$schema";
  @SerializedName(SERIALIZED_NAME_$_SCHEMA)
  private String $schema;

openapi-generator version

v4.0.0

OpenAPI declaration file content or url

Command line used for generation

Steps to reproduce

Related issues/PRs

Suggest a fix

it seems that in the swagger-codegen we won't have the issue.

https://github.com/kubernetes-client/java/blob/dc82549dde1eeea7593203dec788af0fe33685a7/kubernetes/src/main/java/io/kubernetes/client/models/V1beta1JSONSchemaProps.java#L38-L42

Start by reproducing the Java generation from the Kubernetes OpenAPI schema containing the $ref and $schema properties, then compare the generated V1beta1JSONSchemaProps.java output with the linked swagger-codegen version. Done means the generated Java compiles and the escaped property names remain mapped to their schema names.