Fix ExponentialStrategy calculation and update tests

[yamadashy]

Fix: #18

This PR addresses the issue #18 where the ExponentialStrategy was not correctly calculating the wait time for the first attempt. The changes include:

Changes made:

public function getWaitTime($attempt)
{
    return (int) ($this->base * (pow(2, $attempt - 1)));
}

This new implementation ensures that:

• The first attempt (attempt = 1) returns the base wait time
• Subsequent attempts follow the exponential backoff pattern correctly

Example with base time of 100ms:

• 1st attempt: 100ms
• 2nd attempt: 200ms
• 3rd attempt: 400ms
• 4th attempt: 800ms
• ...

Potential impacts:

This change will affect the behavior of the ExponentialStrategy, particularly for the first attempt. Users who were relying on the previous behavior may need to adjust their expectations or configurations.

What I've done:

• PHPUnit locally (all tests passing)

I've tested this locally and everything looks good. Would really appreciate if you could take a look when you have a moment. Let me know if you need anything else!