Removed 'X-Requested-With' from header defaults in angular-seed

I just spent around an hour trying to figure out how to use http get with a simple third party api. The main problem was that this pull request https://github.com/angular/angular.js/pull/1454 removed the 'X-Requested-With' header from angular master but not from the bundled angular in angular-seed. I can imagine a few other people were confused as I was, not realizing that angular-seed doesn't always have the latest changes!