Fix IEBWithTimeout last try for corner cases

[RaviHari]

Fix for: #5

Testing Done:

go test ./...  -coverprofile coverage.txt 
ok  	github.com/keikoproj/inverse-exp-backoff	52.664s	coverage: 100.0% of statements

The issue given here: https://play.golang.org/p/0xpS7ano-vB
is now fixed when pointing to the code in this pr.

go run test.go                                                     
2020-10-08 02:49:20.789311 +0530 IST m=+0.000098405
2020-10-08 02:49:50.789806 +0530 IST m=+30.000406258
2020-10-08 02:50:08.791557 +0530 IST m=+48.002039872
2020-10-08 02:50:19.593109 +0530 IST m=+58.803527428
2020-10-08 02:50:19.79187 +0530 IST m=+59.002287454
All retries done!