Convert VirusScanner class into stateless service #372

The VirusScanner class is only ever instantiated in the VirusScanWorker and in that case, the #clean method is only called once on that instance. Thus this optimisation is only ever exercised in the VirusScanner spec example which I have removed. My motivation for making this change is that I want to convert VirusScanner into a stateless service to bring it into line with S3Storage and services in other GOV.UK apps. This change is a step in that direction.

The #clean method is no longer adding much value and doing this will make it easier to convert this class into a stateless service.

This is a further step towards converting this class into a stateless service.

Previously this method was returning `false` if a virus was detected. I'm converting this class into a stateless service and so I want to avoid having to store `virus_info` when a virus is detected. We can do this by raising the exception and handling it in the VirusScanWorker. Handling the exception in the worker is important, because we want to avoid triggering the Sidekiq retry mechanism. Note that the `virus_info` string is now passed as the exception message and since we send the original exception to GovukError.notify, there's no longer any need to include it in the `extra` Hash.

By removing this last remaining state variable, the class is now a stateless service.

Now that we are explicitly handling VirusScanner::InfectedFile exceptions in VirusScanWorker#perform, I think this comment is less useful and just clutters up the code.

Extract common code into `before` & `let` blocks and use `context` blocks to make the appriopriate changes to the context for each example. This also has the benefit of removing some long lines. I've also replaced the `status` double with an instance double which is more likely to catch problems.

Extract common code into `before` & `let` blocks and use `context` blocks to make the appriopriate changes to the context for each example. This also has the benefit of removing some long lines. I've also replaced the `scanner` double with an instance double which is more likely to catch problems.

Now that the VirusScanner class is a stateless service, we can include it in the standard Services module to bring this app more into line with other GOV.UK apps.

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Convert VirusScanner class into stateless service #372

Convert VirusScanner class into stateless service #372

Commits on Dec 29, 2017