Use correct dialect for escaping functions. (#2593)

Column functions are currently escaped with the
same dialect that is being used by Superset for its
metadata storage.  Instead the dialect of the target
table should be used when escaping column names.

superset/connectors/sqla/models.py

@@ -572,6 +572,7 @@ def fetch_metadata(self):
         M = SqlMetric  # noqa
         metrics = []
         any_date_col = None
+        db_dialect = self.database.get_sqla_engine().dialect
         for col in table.columns:
             try:
                 datatype = "{}".format(col.type).upper()
@@ -603,7 +604,7 @@ def fetch_metadata(self):
                 any_date_col = col.name
 
             quoted = "{}".format(
-                column(dbcol.column_name).compile(dialect=db.engine.dialect))
+                column(dbcol.column_name).compile(dialect=db_dialect))
             if dbcol.sum:
                 metrics.append(M(
                     metric_name='sum__' + dbcol.column_name,