[Java] Bug generating Java model with '_class' property

[menchauser]

Description

This issue is similar to #2841. swagger-codegen may produce invalid code for model like

  test:
    type: object
    properties:
      '_class':
        type: string

Result code contains inelligible overload for method Object.getClass():

    public String getClass() {
        return _class;
    }

Swagger-codegen version

Latest (2.2.2-SNAPSHOT)

[menchauser]

I've implemented simple fix: #4237. That could be not the best solution and I haven't checked all language/framework combinations (except Java clients).