[sql lab] preserve schema through visualize flow (#4742)

* [sql lab] preserve schema through visualize flow

https://github.com/apache/incubator-superset/pull/4696 got tangled
into refactoring views out of views/core.py and onto views/sql_lab.py

This is the same PR without the refactoring.

* Fix lint

superset/assets/spec/javascripts/sqllab/VisualizeModal_spec.jsx

@@ -269,6 +269,7 @@ describe('VisualizeModal', () => {
       chartType: wrapper.state().chartType.value,
       datasourceName: wrapper.state().datasourceName,
       columns: wrapper.state().columns,
+      schema: 'test_schema',
       sql: wrapper.instance().props.query.sql,
       dbId: wrapper.instance().props.query.dbId,
     });