Change backoff semantics in case of initialBackoff

[willdeuschle]

A not uncommon pattern is to use an initial backoff that is larger than the default max backoff. For example:

err = retry.Do(ctx, func() error {
	// do things
}, retry.WithMaxAttempts(5), retry.WithInitialBackoff(15*time.Second), retry.WithMultiplier(1))

I'd expect this to retry 5 times, every 15 seconds. However, because the default backoff is 2s, we end up retrying every two seconds.

The proposed change is to use the larger of max backoff or initial backoff in situations where the current backoff is larger than the max backoff.

Explicitly setting your initial backoff larger than your max backoff is nonsensical anyways, so I haven't been able to come up with an example where this change is detrimental.

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This change is 

[bmoylan]

Would it be better to set maxBackoff = max(maxBackoff, initialBackoff) in Start() after applying the params so we don't do this check every time, given that the logic would be the same? I might consider this semantic more "construction-time" than "runtime"

[willdeuschle]

I considered that and ultimately chose not to do so because it mucks with what is currently a very clean/understandable Start with initialization and then application of option. I'm happy to put it there instead

[bmoylan]

Can you include a comment on the InitialBackoff param explaining that if it's larger than MaxBackoff, it will take precedent?

[willdeuschle]

will do