Handle payment_intent.canceled event

[waiting-for-dev]

Summary

A payment_intent.canceled event is published when canceling a payment intent from the Stripe dashboard. At that point, we need to update the associated Solidus payment to be voided.

However, the webhook could also be received after voiding the payment from the Solidus admin interface, as it updates through Stripe's API. Therefore, we need to do nothing if the payment is already voided.

Closes #181

[1] - https://stripe.com/docs/api/events/types#event_types-payment_intent.canceled

Checklist

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[codecov[bot]]

Codecov Report

Merging #250 (4079188) into master (bad6ed0) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #250   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files          24       24           
  Lines         478      485    +7     
=======================================
+ Hits          477      484    +7     
  Misses          1        1           

Impacted Files	Coverage Δ
lib/solidus_stripe/webhook/event.rb	96.96% <ø> (ø)
...olidus_stripe/webhook/payment_intent_subscriber.rb	100.00% <100.00%> (ø)

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