Get query button working in explorev2 (#1581)

* Get query buttonw working in explorev2

 - Create new endpoint for updating explore viz
 - Send over new form_data when query button is pressed

* Added endpoint test

* Changes based on comments

* Added docstring for endpoint, and query spec

* Remove white space around docstring

tests/core_tests.py

@@ -97,6 +97,25 @@ class CoreTests(SupersetTestCase):
         assert_admin_view_menus_in('Alpha', self.assertNotIn)
         assert_admin_view_menus_in('Gamma', self.assertNotIn)
 
+    def test_update_explore(self):
+        self.login(username='admin')
+        tbl_id = self.table_ids.get('energy_usage')
+        data = json.dumps({
+            'viz_type': 'sankey',
+            'groupby': ['source', 'target'],
+            'metrics': ['sum__value'],
+            'row_limit': 5000,
+            'flt_col_0': 'source',
+            'datasource_name': 'energy_usage',
+            'datasource_id': tbl_id,
+            'datasource_type': 'table',
+            'previous_viz_type': 'sankey'
+        })
+        response = self.client.post('/superset/update_explore/table/{}/'.format(tbl_id),
+            data=dict(data=data))
+        assert response.status_code == 200
+        self.logout()
+
     def test_save_slice(self):
         self.login(username='admin')
         slice_id = self.get_slice("Energy Sankey", db.session).id