Fix another place where terms is defaulted to 0.0

Author: mhucka

src/openfermion/ops/operators/symbolic_operator.py

@@ -480,7 +480,7 @@ def __isub__(self, subtrahend):
         """
         if isinstance(subtrahend, type(self)):
             for term in subtrahend.terms:
-                self.terms[term] = self.terms.get(term, 0.0) - subtrahend.terms[term]
+                self.terms[term] = self.terms.get(term, 0) - subtrahend.terms[term]
                 if self._issmall(self.terms[term]):
                     del self.terms[term]
         elif isinstance(subtrahend, COEFFICIENT_TYPES):