test($interpolate): add a test to address Igor's first concern

which is point #1 in this PR comment https://github.com/angular/angular.js/pull/4556#issuecomment-30961742
also merged Igor's first code review https://github.com/IgorMinar/angular.js/commit/784dd4e9ffc52c834b698ab3d8b212a959ea3fe1