[BUG][typescript-node] Enum params in form body generated as plain strings

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?
• [Optional] Bounty to sponsor the fix (example)

Description

When using a POST form parameter that is an enum, the generated client uses plain string for the argument type.

• form parameter: scope: { type: string, enum: [scope1, scope2, scope3] }
• expected generated method: public async test (scope: ScopeEnum, options: ...): ..., using either:
  • enum ScopeEnum { scope1,scope2, scope3 }
  • type ScopeType = 'scope1' | 'scope2' | 'scope3'
• actual generated method: public async test (scope: string, options: ...): ...

Side question out of curiosity: why are form bodies generated as separate method arguments, while all other request bodies are generated as a single argument typed as a generated model?

openapi-generator version

• openapi-generator-cli-5.0.0-20190811.165202-11.jar
• openapi-generator-cli-4.1.1.jar

OpenAPI declaration file content or url

openapi: 3.0.2

info:
  version: 0.0.1
  title: test

servers:
  - url: http://test

paths:
  /test:
    post:
      summary: test
      operationId: test
      requestBody:
        required: true
        content:
          application/x-www-form-urlencoded:
            schema:
              type: object
              required: [scope]
              properties:
                scope:
                  type: string
                  enum: [scope1, scope2, scope3]
      responses:
        '204':
          description: success

Command line used for generation

java -jar $GENERATOR_JAR generate -i test.yml -g typescript-node -p supportsES6=true -o .

Steps to reproduce

1. Generate the client.
2. Inspect the client code :)

Start by running the provided openapi-generator command with the included OpenAPI 3.0.2 YAML and inspect the generated typescript-node client method for the form parameter. Trace how the application/x-www-form-urlencoded schema and its enum are represented during generation; done means the scope argument uses a generated enum or string-union type instead of plain string.