Correct fields for CancelSubscription

[nryanl]

Corrected CancelSubscription fields for current Stripe API version.

A fix for #394

https://stripe.com/docs/api/subscriptions/cancel

According to the current Stripe API in the link above, there is no "reason" field within cancellation_details any longer. This also aligns with my testing where it did not recognize the parameter. Consider removing this struct in future versions. For now, users can leave this as None.

• Replaced at_period_end with invoice_now
• Added cancellation_details field
• Added prorate field

Checklist

• [x] ran cargo make fmt
• [x] using conventional commits to hightlight user-facing fixes and features

[arlyon]

Thanks for this, running CI

[codecov[bot]]

Codecov Report

Merging #402 (db86ed1) into master (56303e9) will increase coverage by 0.00%. The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master    #402   +/-   ##
======================================
  Coverage    5.90%   5.90%           
======================================
  Files         145     145           
  Lines       22379   22382    +3     
======================================
+ Hits         1321    1322    +1     
- Misses      21058   21060    +2     

Impacted Files	Coverage Δ
src/resources/subscription_ext.rs	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes

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