PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

@antihax
@bvwells

Description of the PR

The OpenAPI 3.0 Spec and Swagger 2.0 Spec define the date-time string format as:

dateTime | string | date-time | As defined by date-time - RFC3339

• Spec 3.0: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md
• Spec 2.0: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md

An example of RFC-3339 date-time format is 2017-07-21T17:32:28Z. Here is the full spec:

• RFC-3339: https://tools.ietf.org/html/rfc3339#section-5.6

The current SDK will create in an incorrectly formatted time for a query parameter. This happens because client.go parameterToString function uses fmt.Sprintf("%v", obj) which produces the following time format which is not a RFC-3339 time:

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

This can be resolved by adding the following code to parameterToString(obj interface{}, collectionFormat string) string.

} else if t, ok := obj.(time.Time); ok {
    return t.Format(time.RFC3339)
}

This approach for testing time has been verified to work and is mentioned here:

https://stackoverflow.com/questions/41483505/golang-check-if-a-data-is-time-time

An example is on Go Playground here: https://play.golang.org/p/Jq16Shapio8

A test is not provided because it appears a date-time format string query parameter does not exist in the current Petstore API.

This is a breaking change since the date time format produced by the Client SDK changes.

More info can be seen here where the issue was encountered:

• client.MessagesApi.ListMessages Parameter [dateFrom] value is invalid grokify/go-ringcentral-client#19
• [Golang] date-time query parameter does not produce a RFC 3339 time per spec definition swagger-api/swagger-codegen#8039

This PR updates client.mustache. Other changed files are from a re-built go-petstore SDK in the sample folder.