Check your $http function parameters - in my case, I was using params instead of data, and that's why it was giving me such error. I was also writing the function a little bit differently.

$http({method: 'POST', url: 'X', params: Y}) - 415 Error

$http({method: 'POST', url: 'X', data: Y}) - No issues

It might also work the other way around - you could be wrongfully using data instead of params.