trigger_rate: fix arithmetic corruption

- because the parent class of the ratebin type is immutable, its out-of-place arithmetic operations don't necessarily return new objects since if the boundaries are identical the object can be re-rused.  but ratebin is not, really, immutable and its own out-of-place arithmetic operations assign values to the internal ._count attribute.  if the parent class' arithmetic methods return references to one or the other of the input arguments then ratebin's methods corrup the input object's count values.
- this patch fixes by ensuring a new object is created in the event the ._count atribute must be assigned a new value.