fix(modal): opening a modal should not change default options

Closes #935

src/modal/modal.js

@@ -231,7 +231,7 @@ angular.module('ui.bootstrap.modal', [])
             };
 
             //merge and clean up options
-            modalOptions = angular.extend(defaultOptions, modalOptions);
+            modalOptions = angular.extend({}, defaultOptions, modalOptions);
             modalOptions.resolve = modalOptions.resolve || {};
 
             //verify options

src/modal/test/modal.spec.js

@@ -399,5 +399,14 @@ describe('$modal', function () {
 
       expect($document).toHaveModalsOpen(2);
     });
+
+    it('multiple modals should not interfere with default options', function () {
+
+      var modal1 = open({template: '<div>Modal1</div>', backdrop: false});
+      var modal2 = open({template: '<div>Modal2</div>'});
+      $rootScope.$digest();
+
+      expect($document).toHaveBackdrop();
+    });
   });
 });