fix($http): throw if url passed is not a string

Throw to prevent hard to debug errors in functions that are
called later.

Fixes #12925
Closes #13444

src/ng/http.js

@@ -918,6 +918,10 @@ function $HttpProvider() {
         throw minErr('$http')('badreq', 'Http request configuration must be an object.  Received: {0}', requestConfig);
       }
 
+      if (!isString(requestConfig.url)) {
+        throw minErr('$http')('badreq', 'Http request configuration url must be a string.  Received: {0}', requestConfig.url);
+      }
+
       var config = extend({
         method: 'get',
         transformRequest: defaults.transformRequest,

test/ng/httpSpec.js

@@ -302,6 +302,13 @@ describe('$http', function() {
       }).toThrowMinErr('$http','badreq', 'Http request configuration must be an object.  Received: /url');
     });
 
+    it('should throw error if the request configuration url is not a string', function() {
+      expect(function() {
+          $http({url: false});
+      }).toThrowMinErr('$http','badreq', 'Http request configuration url must be a string.  Received: false');
+    });
+
+
     it('should send GET requests if no method specified', function() {
       $httpBackend.expect('GET', '/url').respond('');
       $http({url: '/url'});