Revert "Check if value has 'is_nan' before calling it"

This reverts commit 1451c66a8f.

lib/util.py

@@ -113,7 +113,7 @@ class Fiat(object):
         return 'Fiat(%s)'% self.__str__()
 
     def __str__(self):
-        if hasattr(self.value, 'is_nan') and self.value.is_nan():
+        if self.value.is_nan():
             return _('No Data')
         else:
             return "{:.2f}".format(self.value) + ' ' + self.ccy