Merge pull request pmorissette#383 from MDDietz1/master

Prevent security_weights from overwriting values history

Author: timkpaine

bt/backtest.py

@@ -306,9 +306,9 @@ def security_weights(self):
             for m in self.strategy.members:
                 if isinstance(m, bt.core.SecurityBase):
                     if self.strategy.fixed_income:
-                        m_values = m.notional_values
+                        m_values = m.notional_values.copy()
                     else:
-                        m_values = m.values
+                        m_values = m.values.copy()
                     if m.name in vals:
                         vals[m.name] += m_values
                     else: