fixes error when committing after savepoint rollback

when objects had been added in readCurrent

still need to come up with a test

src/ZODB/Connection.py

@@ -762,6 +762,8 @@ class Connection(ExportImport, object):
             self._creating = {}
 
         for oid in creating:
+            if oid in self._readCurrent:
+                self._readCurrent.pop(oid, None)
             o = self._cache.get(oid)
             if o is not None:
                 del self._cache[oid]