RetryNotify does not cancel the backoff via Context Cancel if wrapped in WithMaxTries

[23caterpie]

Currently RetryNotify will not wait a full backoff period if using a backoff WithContext when the context is canceled while it is "backing off." This is good. However, if backoff with context is wrapped in a backoff WithMaxTries, then the context is not used for this anymore. Having a backoff WithMaxTries wrapped in WithContext, however does work as expected.

In order to get the context for use in RetryNotify I propose adding a type check in ensureContext to check for this case. The function would look like such after the edit:

func ensureContext(b BackOff) BackOffContext {
	if cb, ok := b.(BackOffContext); ok {
		return cb
	}
	if tb, ok := b.(*backOffTries); ok {
		return ensureContext(tb.delegate)
	}
	return WithContext(b, context.Background())
}