RACCommand disable at initialization time

I'm implementing a command that should be disabled from the moment that it is initialized until some other event occurs. I found that was impossible:

    RACCommand *command = [[RACCommand alloc] initWithEnabled:[RACSignal return:@NO] signalBlock:^(id _) {
        return [RACSignal empty];
    }];
    expect([command.enabled first]).to.equal(@NO); // fails

This pull makes it so that if enabledSignal sends a value at the time the command is initialized, then `-[RACCommand enabled]` sends that value synchronously before the initializer returns.