[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.