Fix buy_tag not being saved to trade object.

Column is mistakenly excluded because advise_buy() creating this column runs after code detecting presence of buy_tag column.

tests/optimize/__init__.py

@@ -52,4 +52,6 @@ def _build_backtest_dataframe(data):
     # Ensure floats are in place
     for column in ['open', 'high', 'low', 'close', 'volume']:
         frame[column] = frame[column].astype('float64')
+    if 'buy_tag' not in columns:
+        frame['buy_tag'] = None
     return frame