Robert Downey Jr. is the latest in a string of Marvel stars to reach out to a young boy who sustained serious injuries while saving his sister from a dog attack.Mark Ruffalo and Chris Evans were the first to praise young fan Bridger Walker, after his aunt, Nikki Walker started an account on Instagram to explain how her “hero” nephew “took on the attack so that the dog wouldn’t get his sister”.According to his family, Bridger underwent surgery that lasted around two hours and received more than