This should use the same technique as in [`-[UITextField rac_textSignal]`](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/09e17b9b9b62765b925bb241ebd9f03b959cb824/ReactiveCocoaFramework/ReactiveCocoa/UITextField%2BRACSignalSupport.m#L22-30).

Otherwise, this signal could start with a stale value from the time the signal was created, not subscribed.