Issue Description:

When JSON-encoding a dictionary with a Date object, and specifying millisecondsSince1970 for the date encoding strategy, the resulting encoded value for the Date object is a floating point number with some amount of decimal places, instead of an integer representing milliseconds. This doesn't seem correct, I feel like this can cause incompatibility with systems or APIs that don't expect floats for millisecond values (which was the case for my company's Java back-end API).

To reproduce:

let test = ["key": Date()]
let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .millisecondsSince1970
if let result = try? encoder.encode(test) {
    if let backToString = String(data: result, encoding: .utf8) {
        print("JSON: " + backToString)
    }
}

Expected output:

JSON: {"key":1542355551611}

Actual output:

JSON: {"key":1542355551611.2129}