Crash when creating the date formatter

[Edgar Antunes]

I've seen some crashes coming into a production app through Crashlytics at this line:
df = [[NSDateFormatter alloc] init];

I believe they happen because if multiple threads race during the creation of the date formatter, the if clause may not be enough to make sure everything is set up for all the remaining accesses, only a dispatch_once_t is.

This can lead to an incomplete set up for the date formatter.

PR will follow.