Use explicitely the default that was previously used

Author: martinRenou

traittypes/traittypes.py

@@ -200,7 +200,7 @@ class Series(PandasType):
     info_text = 'a pandas series'
     dtype = None
 
-    def __init__(self, default_value=Empty, allow_none=False, dtype=np.float64, **kwargs):
+    def __init__(self, default_value=Empty, allow_none=False, dtype=np.object_, **kwargs):
         if 'klass' not in kwargs and self.klass is None:
             import pandas as pd
             kwargs['klass'] = pd.Series