Suggested extendable $http configuration...

app.js

angular.module('resources', [])

// So we create a new service for each configuration of $http that we want
.factory('myServerAPI', function($http) {
  
  function customRequestTransform(value) { ... }
  function customResponseTransform(value) { ... }

  // Provide a configuration object that corresponds to our specific $http needs
  // for this particular service
  var config = {
    // Provide or override the standard config properties from default
    url: 'https://api.my-server.com',
    headers: {
      common: { ... }
    },
    transformRequest: [customeRequestTransform],
    transformResponse: [customResponseTransform],
    cache: null,
    timeout: 10000,
    // But also override the interceptors for this too!
    interceptors: ['customHttpInterceptor']
  };

  // We use some function such as extend to create a "new" instance of $http that is
  // configured as we want it
  return $http.extend(config);

})

// Even more powerful this new custom $http service exposes an `extend` method too
// So we can further customize our services on a more fine grained level while keeping
// shared config in a reasonble place
.factory('userAPI', function(myServerAPI) {
  
  var config = {
    // By providing a relative URL we can use the base URL from the myServerAPI config
    url: 'auth/user'
    // It reuses the config from myServerAPI by default but we can override bits
    headers: angular.extend(myServerAPI, { get: ... })
  };
  
  return myServerAPI.extend(config);
});