Client Server URLs: support handling an enum with `default` as a value clashing with the default value constant

```yaml
openapi: "3.0.0"
info:
  version: 1.0.0
  title: Server URLs can be optionally generated
servers:
- url: https://{username}.gigantic-server.com:{port}/{basePath}
  description: The production API server
  variables:
    # TODO this conflict will cause broken generated code
    conflicting:
      enum:
        - 'default'
        - '443'
      default: 'default'
```