[Golang] `date-time` query parameter does not produce a RFC 3339 time per spec definition

Description

The type: "string" and format: "date-time" parameter type is supposed to be a RFC 3339 time per the Swagger spec:

https://swagger.io/docs/specification/data-models/data-types/

• date-time – the date-time notation as defined by RFC 3339, section 5.6, for example, 2017-07-21T17:32:28Z

Using a query parameter with type: "string" and format: "date-time" results in a proper time.Time struct, however, the parameterToString() function produces as incorrect string format such as:

2018-01-01 00:00:00 +0000 UTC

This occurs because parameterToString() processes time.Time using fmt.Sprintf("%v", obj) per the code below:

// parameterToString convert interface{} parameters to string, using a delimiter if format is provided.
func parameterToString(obj interface{}, collectionFormat string) string {
	var delimiter string

	switch collectionFormat {
	case "pipes":
		delimiter = "|"
	case "ssv":
		delimiter = " "
	case "tsv":
		delimiter = "\t"
	case "csv":
		delimiter = ","
	}

	if reflect.TypeOf(obj).Kind() == reflect.Slice {
		return strings.Trim(strings.Replace(fmt.Sprint(obj), " ", delimiter, -1), "[]")
	}

	return fmt.Sprintf("%v", obj)
}

Swagger-codegen version

2.3.1

https://github.com/grokify/go-ringcentral/blob/master/codegen/swagger_codegen_version.txt

Swagger declaration file content or url

https://github.com/grokify/go-ringcentral/blob/master/codegen/swagger_spec.yaml

Here is the excerpt with irrelevant parameters omitted:

    /restapi/v1.0/account/{accountId}/extension/{extensionId}/message-store:
      get:
        tags:
          - "Messages"
        summary: "Get Message List"
        operationId: "listMessages"
        consumes:
          - "application/json"
        produces:
          - "application/json"
        parameters:
          -
            name: "dateFrom"
            in: "query"
            description: "The start datetime for resulting messages in ISO 8601 format including timezone, for example 2016-03-10T18:07:52.534Z. The default value is dateTo minus 24 hours"
            required: false
            type: "string"
            format: "date-time"
          -
            name: "dateTo"
            in: "query"
            description: "The end datetime for resulting messages in ISO 8601 format including timezone, for example 2016-03-10T18:07:52.534Z. The default value is current time"
            required: false
            type: "string"
            format: "date-time"

Command line used for generation

$ java -jar swagger-codegen-cli.jar generate -c swagger_codegen_config.json -i swagger_spec.yaml -l go -o client

Steps to reproduce

1. Build the client
2. Make an API call to the endpoint with a time.Time struct.

Related issues/PRs

https://github.com/grokify/go-ringcentral/issues/19

Suggest a fix/enhancement

In parameterToString() add a check for time.Time and format as time.RFC3339 using the else clause below.

https://github.com/grokify/swagger-codegen/commit/83dad17d54778cdf5b28182459cb1345a28afb28

	if reflect.TypeOf(obj).Kind() == reflect.Slice {
		return strings.Trim(strings.Replace(fmt.Sprint(obj), " ", delimiter, -1), "[]")
	} else if t, ok := obj.(time.Time); ok {
		return t.Format(time.RFC3339)
	}

Start at the generated Go client's parameterToString() function and review how time.Time values are converted for query parameters. Reproduce the date-time query call described in the issue, then verify that the resulting value uses RFC 3339 formatting and the client still builds successfully.