[Bug] Exchange Rates defaults to 0.000000

[Gigaion]

Bug:

1. Exchange Rates defaults to 0.000000 if Blesta fails to fetch a currency. This replaces any existing manually entered currency rate.
2. Occurs on latest versions of Blesta 4.12.3 / v5.0.1
3. This may affect Blesta on supported ISO currencies assuming it also returns 0.000000 on corrupted returned values from the exchange rates processor.

Reproduce:

1. Visit /admin/settings/company/currencies/active/
2. Add a new unofficial currency such as BTC (Can be non-crypto).
   1. Set the Exchange Rate to anything other then 0.
3. Visit /admin/settings/company/currencies/setup/ and click "Update Now"
4. Visit /admin/settings/company/currencies/active/ to see all non-ISO currencies converted to 0.000000

Fix:

1. Instead of setting value to 0.000000 for unsupported ISO, the value should remain unchanged from its original value.

Use Case:

1. Digital currencies, Crypto currencies, Gaming currencies, etc.
2. Custom Payment gateways that make use of digital currencies without involving actual cash (E.g. In game currency purchases that use Blesta for purposes of invoicing and purchases of in-game goods).

[Jono]

Looks like this is only a bug for the X Rates integration which is returning a response with an exchange rate of 0.  I've created a task to resolve that here: https://dev.blesta.com/browse/CORE-4145.  In the mean time any off the other processors should work.