[C#] Invalid C# enum generated when allOf is used

Description

We're composing a model with allOf. It has a string enum as one of the properties. The C# code generated by swagger-codegen does not compile. If I remove allOf and explicitly describe the data model, the enum is generated correctly.

Swagger-codegen version

2.3.1

Swagger declaration file content or url

For this json definition, which uses allOf, the generated enum for ActivityType in the Models namespace is not valid C#.

This json file, without allOf, will generate the correct enum.

Command line used for generation

swagger-codegen generate -i ./bad_enum.json -l csharp -o ./output_bad/

Steps to reproduce

1. Run the example command against the bad_enum.json from the gist linked above.
2. Observe that the ActivityType.cs file defines an invalid enum with circular references.
3. Run the example command agains the good_enum.json from the other gist linked above.
4. Observe the correctly defined enum in ActivityType.cs for this output.

Related issues/PRs

Similar to #3338, although in my case the generator is able to output code, but the code will not compile.