Fix TestShouldUseExponentialBackoffIfNoRetryHeader bounds (#530)

The lower bound has an off-by-one error:
2^0 + 2^1 + ... + 2^(n-1) sums up to 2^n-1.

Verified with:

    go test -count=1000 -run TestShouldUseExponentialBackoffIfNoRetryHeader ./httputil -v

Author: afq984

httputil/httputil_test.go

@@ -176,7 +176,8 @@ func TestShouldUseExponentialBackoffIfNoRetryHeader(t *testing.T) {
 	}
 
 	delta := time.Millisecond * 100
-	lower := (1 << uint(retries)) * time.Second
+	// exponential backoff: 1s + d1, 2s + d2, 4s + d3, 8s + d4 with dx being a random value in [0ms, 500ms]
+	lower := (1<<uint(retries))*time.Second - time.Second
 	upper := lower + time.Duration(retries*500)*time.Millisecond
 	if total < lower-delta || upper+delta < total {
 		t.Fatalf("Expected a total sleep time between %s and %s (%d retries, exponential backoff with fuzzing), but waited %s instead", lower, upper, retries, total)