Tweaks about refunds sync

[waiting-for-dev]

Summary

This PR is a result of a post-merge review on #281.

• Scope searching refunds to the payment method when syncing
• Hide concrete details about the needed metadata to skip refunds sync
• Use more standard Ruby

Checklist

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• [x] I have written a thorough PR description.
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[codecov[bot]]

Codecov Report

Merging #290 (a00ef98) into master (a1b5866) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #290   +/-   ##
=======================================
  Coverage   99.62%   99.63%           
=======================================
  Files          30       30           
  Lines         540      547    +7     
=======================================
+ Hits          538      545    +7     
  Misses          2        2           

Impacted Files	Coverage Δ
app/models/solidus_stripe/gateway.rb	100.00% <ø> (ø)
lib/solidus_stripe/refunds_synchronizer.rb	100.00% <100.00%> (ø)

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