`YAMLEncoder` outputs a result that is delayed by 1 second when encoding `Date` with nanoseconds greater than 999499997

[norio-nomura]

DateFormatter doesn't support time more precise than milliseconds, and smaller digits will be rounded to milliseconds.
Since DateComponents having a value of 999499997 or greater in nanosecond generates Date in which nanosecond is 999500036 or greater, DateFormatter rounds up and advances the result by 1 second.

POC:

import Foundation

let iso8601WithFractionalSecondFormatter: DateFormatter = {
    let formatter = DateFormatter()
    formatter.locale = Locale(identifier: "en_US_POSIX")
    formatter.dateFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss.SSS'Z"
    formatter.timeZone = TimeZone(secondsFromGMT: 0)
    return formatter
}()

let gregorian = Calendar(identifier: .gregorian)
let utc = TimeZone(identifier: "UTC")!
let components = gregorian.dateComponents(in: utc, from: Date())

func check(from components: DateComponents, with nanosecond: Int) {
    print("--")
    print("Dircted nanosecond: \(nanosecond)")
    var components = components
    components.nanosecond = nanosecond
    let date = components.date!
    print("Actual nanosecond : \(gregorian.dateComponents(in: utc, from: date).nanosecond!)")
    print("Date: \(iso8601WithFractionalSecondFormatter.string(from: date))")
}

check(from: components, with: 999499976)
check(from: components, with: 999499977)

Result:

--
Dircted nanosecond: 999499976
Actual nanosecond : 999499917
Date: 2019-06-25T14:35:36.999Z
--
Dircted nanosecond: 999499977
Actual nanosecond : 999500036
Date: 2019-06-25T14:35:37.000Z

We should drop nanosecond from Date before passing to DateFormatter when using nanosecond precision.

[norio-nomura]

The Date Formatting Guide said:

The format string uses the format patterns from the Unicode Technical Standard #35.

And the Unicode Technical Standard #35 said:

Field	Sym.	No.	Example	Description
second	S	1..n	3456	Fractional Second - truncates (like other time fields) to the count of letters. (example shows display using pattern SSSS for seconds value 12.34567)

DateFormatter rounds sub-millisecond and also caused # 192.
I filed this behavior and document inconsistency as a bug: https://bugs.swift.org/browse/SR-11049