Fix cache issue for currencies with no symbol

[evgk]

Using getCurrencySymbol() leads to a bug for currencies where there is no currency symbol - cache ends up being non-unique. Using currency code for caching is a more foolproof way.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[orlangur]

Please adopt corresponding test by fixing existing case and also add a new one, with empty currency symbol.

As always, please do amend commit and force push.

[evgk]

@orlangur, I fixed the test and submitted an issue with instructions on how to reproduce the problem. Regarding a separate test with an empty currency-symbol, doesn't seem like it makes sense in the context of this fix, since the whole idea is to stop using currency-symbol and use currency-code for caching.

[orlangur]

@evgk ok, looks like there is quite low probability of regression in this place.

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-826 has been created to process this Pull Request

[magento-engcom-team]

@evgk thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.