fix($resource): prevent URL template from collapsing into an empty string

if url template would result in an empty string, we should make a request
to '/' instead.

Closes angular#5455
Closes angular#5493

Author: Gias Kay Lee

src/ngResource/resource.js

@@ -401,7 +401,7 @@ angular.module('ngResource', ['ng']).
         });
 
         // strip trailing slashes and set the url
-        url = url.replace(/\/+$/, '');
+        url = url.replace(/\/+$/, '') || '/';
         // then replace collapse `/.` if found in the last URL path segment before the query
         // E.g. `http://url.com/id./format?q=x` becomes `http://url.com/id.format?q=x`
         url = url.replace(/\/\.(?=\w+($|\?))/, '.');

test/ngResource/resourceSpec.js

@@ -150,6 +150,13 @@ describe("resource", function() {
     R.get({a:6, b:7, c:8});
   });
 
+  it('should not collapsed the url into an empty string', function() {
+    var R = $resource('/:foo/:bar/');
+
+    $httpBackend.when('GET', '/').respond('{}');
+
+    R.get({});
+  });
 
   it('should support escaping colons in url template', function() {
     var R = $resource('http://localhost\\:8080/Path/:a/\\:stillPath/:b');