[SWIFT4] CodableHelper's encode() does not use dateFormatter property

Created by: manhhavu

Description

The current Swift4's CodeableHelper's encode() method does not into account the class' property dateFormatter. The consequence is that we are unable to customize the encoding format for date.

openapi-generator version

3.2.3

Suggest a fix/enhancement

Take into account the property dateFormatter, which is already the case for decode() method. For example:

if let df = self.dateformatter {
    encoder.dateEncodingStrategy = .formatted(df)
} else {
    encoder.dataEncodingStrategy = .base64
    let formatter = DateFormatter()
    formatter.calendar = Calendar(identifier: .iso8601)
    formatter.locale = Locale(identifier: "en_US_POSIX")
    formatter.timeZone = TimeZone(secondsFromGMT: 0)
    formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"
    encoder.dateEncodingStrategy = .formatted(formatter)
}

I'm happy to create a PR if you'd like 😁