allOf needs `$ref` first

Description

When generating a library with an allOf construct in a model definition, the codegen seems to have problems if the first array item is an inline object vs. a model reference

Swagger-codegen version

2.1.6

Command line used for generation

Occurs on java with this trace:

Exception in thread "main" java.lang.RuntimeException: Could not process model 'XXXX'
    at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:244)
    at io.swagger.codegen.cmd.Generate.run(Generate.java:223)
    at io.swagger.codegen.SwaggerCodegen.main(SwaggerCodegen.java:36)
Caused by: java.lang.ClassCastException: io.swagger.models.ModelImpl cannot be cast to io.swagger.models.RefModel
    at io.swagger.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:955)
    at io.swagger.codegen.languages.JavaClientCodegen.fromModel(JavaClientCodegen.java:541)
    at io.swagger.codegen.DefaultGenerator.processModels(DefaultGenerator.java:799)
    at io.swagger.codegen.DefaultGenerator.generate(DefaultGenerator.java:237)

Start at DefaultGenerator.generate and trace the stack into DefaultCodegen.fromModel and JavaClientCodegen.fromModel. Reproduce Java generation with an allOf whose inline object precedes a model reference; done means generation no longer fails with the reported ModelImpl-to-RefModel ClassCastException.