fix($resource): allow falsy values in URL parameters

Close #1212

when a param value was 0 (or false) it was ignored and removed from url.
after this fix that only happens if the value is undefined or null.

src/ngResource/resource.js

@@ -285,7 +285,8 @@ angular.module('ngResource', ['ng']).
 
         params = params || {};
         forEach(this.urlParams, function(_, urlParam){
-          if (val = (params[urlParam] || self.defaults[urlParam])) {
+          val = params.hasOwnProperty(urlParam) ? params[urlParam] : self.defaults[urlParam];
+          if (isDefined(val) && val !== null) {
             encodedVal = encodeUriSegment(val);
             url = url.replace(new RegExp(":" + urlParam + "(\\W)", "g"), encodedVal + "$1");
           } else {

test/ngResource/resourceSpec.js

@@ -51,14 +51,22 @@ describe("resource", function() {
   it('should ignore slashes of undefinend parameters', function() {
     var R = $resource('/Path/:a/:b/:c');
 
-    $httpBackend.when('GET').respond('{}');
-    $httpBackend.expect('GET', '/Path');
-    $httpBackend.expect('GET', '/Path/1');
-    $httpBackend.expect('GET', '/Path/2/3');
-    $httpBackend.expect('GET', '/Path/4/5');
-    $httpBackend.expect('GET', '/Path/6/7/8');
+    $httpBackend.when('GET', '/Path').respond('{}');
+    $httpBackend.when('GET', '/Path/0').respond('{}');
+    $httpBackend.when('GET', '/Path/false').respond('{}');
+    $httpBackend.when('GET', '/Path').respond('{}');
+    $httpBackend.when('GET', '/Path/').respond('{}');
+    $httpBackend.when('GET', '/Path/1').respond('{}');
+    $httpBackend.when('GET', '/Path/2/3').respond('{}');
+    $httpBackend.when('GET', '/Path/4/5').respond('{}');
+    $httpBackend.when('GET', '/Path/6/7/8').respond('{}');
 
     R.get({});
+    R.get({a:0});
+    R.get({a:false});
+    R.get({a:null});
+    R.get({a:undefined});
+    R.get({a:''});
     R.get({a:1});
     R.get({a:2, b:3});
     R.get({a:4, c:5});