Fix has_table method (apache#3741)

Dialect's has_table method requires connection as the first argument, not engine (https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/engine/interfaces.py#L454). Instead, we can use engine's has_table method that handles the connection for us (https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/engine/base.py#L2141). Alternatively, we could call engine.dialect.has_table(engine.connect(), ...).

superset/models/core.py

@@ -788,8 +788,8 @@ def get_perm(self):
 
     def has_table(self, table):
         engine = self.get_sqla_engine()
-        return engine.dialect.has_table(
-            engine, table.table_name, table.schema or None)
+        return engine.has_table(
+            table.table_name, table.schema or None)
 
     def get_dialect(self):
         sqla_url = url.make_url(self.sqlalchemy_uri_decrypted)