title: How to specify headers parameter for custom Angular $resource action
body: <p>The following works fine, but I am thinking this modifies the $httpProvider globally, which isn't what I want.</p>
  
  <pre><code>angular.module('SessionService', ['ngResource'])
      .config(function($httpProvider){
          $httpProvider.defaults.headers.post['Content-Type'] = 'application/x-www-form-urlencoded; charset=UTF-8'
      })
      .factory('Login', function($resource){
          var resource = $resource('/adminui/login',{},{
              post:{
                  method:"POST",
                  isArray:false
              },
          });
          return resource;
      })
  LoginCtrl = function($scope,Login) {
      $scope.login = function(){
          Login.post($.param({user:$scope.user.username,password:$scope.user.password}),$.noop,$.noop)
      }
  }
  </code></pre>
  
  <p>Is there anyway to do this instead?</p>
  
  <pre><code>...
      .factory('Login', function($resource){
          var resource = $resource('/adminui/login',{},{
              post:{
                  method:"POST",
                  isArray:false,
                  headers:{'Content-Type':'application/x-www-form-urlencoded; charset=UTF-8'} // ignored
              },
          });
          return resource;
      })
  </code></pre>
  
  <p>The "headers" parameter seems to be ignored. the request is still</p>
  
  <p><code>Content-Type:application/json;charset=UTF-8</code> </p>
  
  <p>Is my value for headers ok?</p>