Add case for calculating rate of zero time delta

This was not seen in real life transfers, but for the functional
tests, the time deltas between reads were infitesimally small
causing the rate to be calculate with a time of 0 and throw
a ZeroDivisionError

s3transfer/bandwidth.py

@@ -401,7 +401,14 @@ def record_consumption_rate(self, amt, time_at_consumption):
         self._last_time = time_at_consumption
 
     def _calculate_rate(self, amt, time_at_consumption):
-        return amt / (time_at_consumption - self._last_time)
+        time_delta = time_at_consumption - self._last_time
+        if time_delta <= 0:
+            # While it is really unlikley to see this in an actual transfer,
+            # we do not want to be returning back a negative rate or try to
+            # divide the amount by zero. So instead return back an infinite
+            # rate as the time delta is infinitesimally small.
+            return float('inf')
+        return amt / (time_delta)
 
     def _calculate_exponential_moving_average_rate(self, amt,
                                                    time_at_consumption):

tests/unit/test_bandwidth.py

@@ -456,3 +456,10 @@ def test_get_projected_rate(self):
         self.assertEqual(projected_rate, 0.96)
         self.rate_tracker.record_consumption_rate(1, 3)
         self.assertEqual(self.rate_tracker.current_rate, projected_rate)
+
+    def test_get_projected_rate_for_same_timestamp(self):
+        self.rate_tracker.record_consumption_rate(1, 1)
+        self.assertEqual(
+            self.rate_tracker.get_projected_rate(1, 1),
+            float('inf')
+        )