BUG: Run index through type coercion after numeric operations.

Closes pandas-dev#9966.

Author: josteinbf

pandas/core/index.py

@@ -2531,7 +2531,7 @@ def _evaluate_numeric_binop(self, other):
                 if reversed:
                     values, other = other, values
 
-                return self._shallow_copy(op(values, other))
+                return Index(op(values, other), **self._get_attributes_dict())
 
             return _evaluate_numeric_binop