BUG: Aggregate Equity on 'last' when plot resampling

Fixes kernc/backtesting.py#162

Thanks @eervin123

Author: goblincomet

backtesting/lib.py

@@ -60,7 +60,7 @@
 """
 
 _EQUITY_AGG = {
-    'Equity': 'mean',
+    'Equity': 'last',
     'DrawdownPct': 'max',
     'DrawdownDuration': 'max',
 }