Generate PathParamSerializers for enum in javadsl

[ignasi35]

Generated Descriptors for javadsl overlooked path param serializers.

The scripted test for petstore-java for example compiles but won't run because at least the following path param serializers for the StatusEnum query param in findPetsByStatus is missing.

.withPathParamSerializer(
                StatusEnum.class, 
                PathParamSerializers.required(
                        "Status",
                        StatusEnum::fromValue,
                        StatusEnum::toString)
        );

[ignasi35]

related to #20