[billing] Cancel non-positive invoices on charge

[guicassolato]

What this PR does / why we need it

What the title says, it ensures invoices with non-positive cost are cancelled when charged.

Which issue(s) this PR fixes

Closes THREESCALE-1420

Verification steps

• Subscribe to a paid plan (new application)
• Before billing is run, change to a free plan (this will generate a non-positive invoice)
• Charge the invoice
• The invoice should not be charged and its state should change to 'cancelled'

[codecov]

Codecov Report

Merging #491 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #491      +/-   ##
==========================================
+ Coverage    92.7%   92.73%   +0.02%     
==========================================
  Files        2344     2342       -2     
  Lines       75695    75621      -74     
==========================================
- Hits        70173    70127      -46     
+ Misses       5522     5494      -28

Impacted Files	Coverage Δ
app/models/invoice.rb	92.61% <100%> (+0.02%)	⬆️
test/unit/invoice_test.rb	100% <100%> (ø)	⬆️
...workers/destroy_all_deleted_objects_worker_test.rb	100% <0%> (ø)	⬆️
test/test_helpers/sidekiq.rb	100% <0%> (ø)	⬆️
config/environments/test.rb	100% <0%> (ø)	⬆️
config/jobs.rb	60% <0%> (ø)	⬆️
test/unit/tasks/services_test.rb	100% <0%> (ø)	⬆️
lib/gitlab/testing/request_inspector_middleware.rb
lib/gitlab/testing/request_blocker_middleware.rb

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[josemigallas]

What if not reason? Does it return null.. ?

[guicassolato]

it does

[Martouta]

maybe it would be clearer with:

return unless (reason = CONDITIONS_TO_CHARGE.find { |condition| !method("#{condition}?").call })
I18n.t(reason, scope: %i[invoices reasons_cannot_charge])

But either way is fine 👍

[guicassolato]

I didn't even have to touch this, TBH. Just did to make it shorter. It's Ruby language anyway.

[Martouta]

Makes sense to me but I don't know much about Billing so it would be good if @hallelujah can review it as well 😄