Merge pull request ReactiveCocoa#25 from barakwei/feature/fix-nullability

RACSubject: Fix nullability annotation to match header.

Author: StatusReport

ReactiveObjC/RACSubject.m

@@ -90,7 +90,7 @@ - (void)enumerateSubscribersUsingBlock:(void (^)(id<RACSubscriber> subscriber))b
 
 #pragma mark RACSubscriber
 
-- (void)sendNext:(id)value {
+- (void)sendNext:(nullable id)value {
 	[self enumerateSubscribersUsingBlock:^(id<RACSubscriber> subscriber) {
 		[subscriber sendNext:value];
 	}];