Thread-safe date formatter

[ghost]

When handling multiple threads, the shared date formatter can provide a result different than expected due to one thread having a different date_format than another one and being performed at the same time.

[malcommac]

I've created a per-thread cache for NSDateFormatter.
It was published with 5d5f25a and available via cocoapods as 1.0.5.
Basically I save and retrive the object from inside the NSThread dictionary; this ensure you to get the instance relative to that thread called only once. It should be enough, tell me if you have problems.

private class func cachedObjectInCurrentThread<T: AnyObject>(key: String, create: () -> T) -> T {
    if let threadDictionary = NSThread.currentThread().threadDictionary as NSMutableDictionary? {
        if let cachedObject = threadDictionary[key] as! T? {
            return cachedObject
        } else {
            let newObject = create()
            threadDictionary[key] = newObject
            return newObject
        }
    } else {
        assert(false, "Current NSThread dictionary is nil. This should never happens, we will return a new instance of the object on each call")
        return create()
    }
}

private class func localThreadDateFormatter() -> NSDateFormatter {
    return NSDate.cachedObjectInCurrentThread("com.library.swiftdate.dateformatter") {
        let dateFormatter = NSDateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"
        return dateFormatter
    }
}