retryIf should be called before onRetry

[pallavJha]

• The func onRetry is called before checking if the retryableFunc should be retried. The func retryIf, called after onRetry, tells whether the retry should be performed or not. retryIf should be called before onRetry.
• fixes The func onRetry should not be called when retryIf returns false #20

[codecov-io]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #21   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files           2        2           
  Lines          70       70           
=======================================
  Hits           68       68           
  Misses          2        2

Impacted Files	Coverage Δ
retry.go	95.83% <100%> (ø)	⬆️

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[JaSei]

thank you for you PR