Fix parsing of non-string enum values

[kevinoid]

JSON Schema v4 allows enum to contain values of any type, explicitly including null. Swagger v2.0 adopts this definition without change (as far as I am aware) but the parser previously parsed any non-string values as null. This PR fixes this to return the string representation of non-string primitive values and to warn about unsupported non-primitive values and add test cases to cover these.

For reference, I discovered this issue when attempting to run swagger-codegen against a Swagger declaration with an integer enumeration, which produced the following error:

Exception in thread "main" java.lang.RuntimeException: Could not process model 'MyEnum'.Please make sure that your schema is correct!
        at io.swagger.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:290)
        at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:647)
        at io.swagger.codegen.cmd.Generate.run(Generate.java:223)
        at io.swagger.codegen.SwaggerCodegen.main(SwaggerCodegen.java:41)
Caused by: java.lang.NullPointerException
        at io.swagger.codegen.DefaultCodegen.postProcessModelsEnum(DefaultCodegen.java:229)
        at io.swagger.codegen.languages.JavaClientCodegen.postProcessModelsEnum(JavaClientCodegen.java:316)
        at io.swagger.codegen.languages.AbstractJavaCodegen.postProcessModels(AbstractJavaCodegen.java:719)
        at io.swagger.codegen.DefaultGenerator.processModels(DefaultGenerator.java:919)
        at io.swagger.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:285)
        ... 3 more

[fehguy]

I believe this is not legal in the spec. @webron can you confirm?

[kevinoid]

@fehguy In the JSON Schema spec or the Open API Spec? Could you provide a link or an explanation for why you think so?

[fehguy]

OpenAPI spec is what the parser works towards, that is the bible for us. As I said, @webron can help point to the language or clarify any ambiguity

[webron]

So there are a few parts in the spec that are unfortunately not documented properly, however, not everything that @kevinoid provided is actually supported.

Since null is not a valid value by the spec, it is not valid in enums.
Since mixed types are not supported by the spec, it can be inferred that mixed types for enums are also not supported. This is not clear in the spec, and will be clarified in the next version.

That said, the spec does support homogeneous enums, regardless of the type. So enums of strings, integers, numbers (and, well, even booleans) are supported.

[kevinoid]

Thanks @webron, I'll update the test to remove MixedEnum. Could you point me to where in the spec it says that null is not a valid value or that mixed types are not supported? I must have overlooked that part somehow.

[webron]

The lack of support for null is a bit hidden, but is suggested in https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#parameterType. While the Schema Object does not document the same restriction, that was the intent.

As mentioned in the original comment, the spec does not document that mixed types are not supported, but they are not.

[kevinoid]

Ah, I see. That explains how I missed it. Thanks @webron!

Also, PR updated. Any remaining issues for me to address?