An example of an Angular $http interceptor to store and reuse headers between requests

karma.conf.js

// Karma configuration
// Generated on Thu Feb 25 2016 23:29:01 GMT+0100 (CET)

module.exports = function(config) {
  config.set({

    // base path that will be used to resolve all patterns (eg. files, exclude)
    basePath: '',


    // frameworks to use
    // available frameworks: https://npmjs.org/browse/keyword/karma-adapter
    frameworks: ['jasmine'],


    // list of files / patterns to load in the browser
    files: [
      'node_modules/angular/angular.js',
      '*.js'
    ],


    // list of files to exclude
    exclude: [
    ],


    // preprocess matching files before serving them to the browser
    // available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
    preprocessors: {
    },


    // test results reporter to use
    // possible values: 'dots', 'progress'
    // available reporters: https://npmjs.org/browse/keyword/karma-reporter
    reporters: ['progress'],


    // web server port
    port: 9876,


    // enable / disable colors in the output (reporters and logs)
    colors: true,


    // level of logging
    // possible values: config.LOG_DISABLE || config.LOG_ERROR || config.LOG_WARN || config.LOG_INFO || config.LOG_DEBUG
    logLevel: config.LOG_INFO,


    // enable / disable watching file and executing tests whenever any file changes
    autoWatch: true,


    // start these browsers
    // available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
    browsers: ['PhantomJS'],


    // Continuous Integration mode
    // if true, Karma captures browsers, runs the tests and exits
    singleRun: false,

    // Concurrency level
    // how many browser should be started simultaneous
    concurrency: Infinity
  })
}

ng-header-reuse-interceptor.js

function Interceptor()
{
  var headers;
  
  this.request = function(request) {
    if (! headers) {
      headers = request.headers;
    } 
    
    request = angular.copy(request);
    request.headers = angular.copy(headers);
    
    return request;
  }
}

ng-header-reuse-interceptor.test.js

describe("Interceptor.request", function() {
  var SUT;
  
  beforeEach(function(){
    SUT = new Interceptor;
  });
  
  it("should return the same header on subsequent requests", function(){
    var firstRequest = {
      headers: {
        foo: 'bar'
      }
    };
    var secondRequest = {};
    
    SUT.request(firstRequest);
    var interceptedSecondRequest = SUT.request(secondRequest);
    
    expect(interceptedSecondRequest.headers).toEqual(firstRequest.headers);
  });
  
  it("should not modify the requests but copy them", function(){
    var request = {};
    
    var interceptedRequest =  SUT.request(request);
    
    expect(interceptedRequest).not.toBe(request);
  });
});

package.json

{
  "dependencies": {
    "angular": "^1.5.0",
    "jasmine-core": "^2.4.1",
    "karma": "^0.13.21",
    "karma-jasmine": "^0.3.7",
    "karma-phantomjs-launcher": "^1.0.0",
    "phantomjs-prebuilt": "^2.1.4"
  },
  "scripts": {
    "test": "karma start"
  }
}

@salusinarduis i've added some boilerplate to let you run the tests and play with it

just clone the gist and run "npm install && npm test"

os.config(function($httpProvider){
$httpProvider

.interceptors.push(function($q, ipCookie) {
  return {
     'request': function(config) {
   // same as above
        },

'response': function(response) {
// same as above
}
}

});