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

Created by: quezak

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

Generate the client.
Make the test call to any server, log the error and check the request body.