Fixbug for Backoff when many attempts: "UnsupportedError: Unsupported operation: Infinity or NaN toInt"

[fzyzcjy]

Hi thanks for the lib! For the backoff, when jitter is nonzero and there are many many attempts, the following error occurs:

UnsupportedError: Unsupported operation: Infinity or NaN toInt
  File "double.dart", line 219, in _Double.toInt
  File "double.dart", line 183, in _Double.floor

caused by the line of var deviation = (rand * jitter * ms).floor();, because ms is Infinity and floor cannot be called on it.

Thus I propose a fix to it :)

[jumperchen]

Thanks for the contribution.

[fzyzcjy]

You are welcome!