feat(http): add data parameter to headers functions

This pull request adds to $http header functions a data parameter, so headers can use object properties to be built:

Example (in a $resource) :
<pre>
...
                  login: {
                    url: './api/login.json',
                    method: 'POST',
                    headers : {
                        'Authorization': function(data) {
                          return 'Basic ' + btoa(data.user + ':' + data.pass);
                        }
                    },
...
</pre>