[IMP] web: rpc: more permissive API for read_group

This rev. allows to specify some kwargs of read_group directly in
an Object given to kwargs(), instead of explicitly specify them
using the corresponding method (e.g. to specify the context in the
kwargs instead of by calling .withContext().

addons/web/static/src/js/core/rpc.js

@@ -264,7 +264,7 @@ var ReadGroupRPCBuilder = SearchRPCBuilder.extend({
      * @returns {Object}
      */
     _getParams: function () {
-        var kwargs = _.extend({}, this._kwargs, {
+        var kwargs = _.defaults({}, this._kwargs, {
             context: this._context || {},
             domain: this._domain || [],
             fields: this._fields,

addons/web/static/test/core/rpc_tests.js

@@ -208,6 +208,38 @@ QUnit.module('core', {}, function () {
             .exec();
     });
 
+    QUnit.test('read_group with kwargs', function (assert) {
+        assert.expect(2);
+        var query = createQuery({
+            model: 'partner',
+            method: 'read_group',
+            mockRPC: function (route, args) {
+                assert.deepEqual(args, {
+                    args: [],
+                    kwargs: {
+                        context: {abc: 'def'},
+                        domain: ['a', '=', 1],
+                        fields: ['name'],
+                        groupby: ['product_id'],
+                        lazy: true,
+                        orderby: false,
+                    },
+                    method: 'read_group',
+                    model: 'partner',
+                }, "should have correct args");
+                assert.equal(route, '/web/dataset/call_kw/partner/read_group',
+                    "should call correct route");
+            },
+        });
+        query
+            .withDomain(['a', '=', 1])
+            .withFields(['name'])
+            .groupBy(['product_id'])
+            .kwargs({context: {abc: 'def'}})
+            .lazy(true)
+            .exec();
+    });
+
     QUnit.test('search_read with no domain, nor fields', function (assert) {
         assert.expect(2);
         createQuery({