Test to verify that in a RACSignal subscription chain, the completed event on a signal downstream of a `+merge:` invokes the disposable of an upstream signal. (ReactiveCocoa)

DisposeSignalUpstreamOfMerge.m

	it(@"should propagate disposal to signals upstream of a +merge: to which it is subscribed when it completes", ^{
		__block BOOL disposed1 = NO;
		__block BOOL disposed2 = NO;
		RACSignal *upstream1 = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> _) {
			return [RACDisposable disposableWithBlock:^{
				disposed1 = YES;
			}];
		}];
		RACSignal *upstream2 = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> _) {
			return [RACDisposable disposableWithBlock:^{
				disposed2 = YES;
			}];
		}];
		RACSubject *trigger = [RACSubject subject];
		RACSignal *merged = [RACSignal merge:@[upstream1, upstream2]];
		RACSignal *downstream = [merged takeUntil:trigger];
		[downstream subscribeNext:^(id _) { }];
		expect(disposed1).to.beFalsy();
		expect(disposed2).to.beFalsy();
		
		[trigger sendNext:RACUnit.defaultUnit];
		
		expect(disposed1).to.beTruthy();
		expect(disposed2).to.beTruthy();
	});