Ensure lowercase column names before column differential check

This change fixes Issue airflow-plugins#18 by ensuring column names are lowercased before
column differential check.

operators/s3_to_redshift_operator.py

@@ -187,7 +187,7 @@ def reconcile_schemas(self, schema, pg_hook):
             """.format(self.redshift_schema, self.table)
 
         pg_schema = dict(pg_hook.get_records(pg_query))
-        incoming_keys = [column['name'] for column in schema]
+        incoming_keys = [column['name'].lower() for column in schema]
         diff = list(set(incoming_keys) - set(pg_schema.keys()))
         print(diff)
         # Check length of column differential to see if any new columns exist