[Scala] Collection parameters not serializing properly

Description

Collection parameters get serialized via toString(), resulting in URL parameters like List%28123%29 for List(123).

Swagger-codegen version

2.3.0

Swagger declaration file content or url

{
  "swagger": "2.0",
  "info": {
    "version": "1.0.0",
    "title": "Collection parameter issue",
    "description": "An endpoint that takes a collection as a parameter",
    "termsOfService": "http://swagger.io/terms/"
  },
  "host": "example.com",
  "basePath": "/api",
  "schemes": [
    "http"
  ],
  "consumes": [
    "application/json"
  ],
  "produces": [
    "application/json"
  ],
  "paths": {
    "/hi": {
      "get": {
        "operationId": "hi",
        "parameters": [
          {
            "collectionFormat": "csv",
            "description": "Comma separated list of integers",
            "in": "query",
            "items": {
              "type": "integer"
            },
            "name": "numbers",
            "required": true,
            "type": "array"
          }
        ],
        "responses": {
          "200": {
            "description": "Success"
          }
        },
        "tags": [
          "hi"
        ]
      }
    }
  }
}

Command line used for generation

java -jar swagger-codegen.jar generate --lang scala -i collection-issue.json

Steps to reproduce

val api = new HiApi(defBasePath="http://localhost:8080")
val result = api.hi(List(1, 2, 3))

Start a simple webserver on 8080 python -m SimpleHTTPServer 8080

Observe:

127.0.0.1 - - [13/Dec/2017 15:29:19] "GET /hi?numbers=List%281%2C%202%2C%203%29 HTTP/1.1" 404 -

Related issues/PRs

#6540 implemented support for collections, but I don't think it ever worked properly, at least for parameters. Notably, adding an escape() function for List[String] is useless when invokeApi's queryParams is Map[String, String].

Suggest a fix/enhancement

Serialization should be addressed inside api.mustache, replacing/augmenting param.toString call with a call to escape() instead. Also see java's parameterToString() as another valid approach.

Start in the Scala api.mustache template and inspect how query parameters are converted before invokeApi receives its queryParams map. Reproduce the issue with the supplied collection-issue.json, generation command, and Scala call, then compare the approach with Java's parameterToString(). Done means a collection such as List(1, 2, 3) produces the intended serialized query rather than List%281%2C%202%2C%203%29.