[BUG][Elm] enum inside an object generates wrong encoder/decoder

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
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Description

Elm generator doesn't render decoder/encoder for an enum inside an object.

openapi-generator version

5.0.0 branch, 4th Feb 2020

OpenAPI declaration file content or url

Defining the following in an object type:

 state:
          type: string
          enum: [Trial, Subscribed, Overdue, Canceled, Archived, NoSubscription]

Command line used for generation

Steps to reproduce

Will result to |> decode "state"

The output is has no decoder, I've narrowed it down to dataType in modules/openapi-generator/src/main/resources/elm/fieldDecoder.mustache.

Note: if I reference schema definition with $ref it works as expected.

Start with modules/openapi-generator/src/main/resources/elm/fieldDecoder.mustache and compare the inline enum case with the working $ref case. Reproduce generation from the inline enum schema and inspect the generated decoder and encoder. Done means the enum inside an object receives the expected generated handling instead of an empty decoder expression.