FOXBOROUGH, Mass. — A 6-year-old Massachusetts boy who received thousands of birthday cards from strangers last year has died after battling a rare inoperable brain tumor.The death of Danny Nickerson of Foxborough was announced by his family on their Danny's Warriors Facebook page. His mother's post said Danny died Friday at home after "a courageous 18 month battle."The post says "He is now cancer free and running and playing and laughing with God and all of the other precious children." It asks for prayers.