[BUG][typescript-node] invalid handling of array param in forms, `encoding` options ignored

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 an array POST form parameter, the generated client serializes it improperly, and ignores the encoding settings in the request body specification.

• parameter: scope, an array of strings
• sample call: api.test(['scope1', 'scope2'])
• expected request body with encoding.scope.explode: true: scope=scope1&scope=scope2
• expected request body with encoding.scope.explode: false: scope=scope1,scope2
• actual request body: scope%5B0%5D=scope1&scope%5B1%5D=scope2, which is the urlencoded form of scope[0]=scope1&scope[1]=scope2...

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: array
                  items:
                    type: string
                    enum: [scope1, scope2, scope3]
            encoding:
              scope:
                # btw: what is `contentType` intended to do here for a string param?
                # cli complains if it's not present, but the value seems to be ignored anyway...
                contentType: application/x-www-form-urlencoded
                style: form
                explode: false
      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. Make the test call to any server, log the error and check the request body.

Start by generating the typescript-node client with the supplied OpenAPI YAML and command, then inspect the generated form-request handling. Reproduce the call with scope=['scope1','scope2'] for both explode settings and compare the request body with the expected scope=scope1&scope=scope2 or scope=scope1,scope2 output.