Currency-Conv module caching works only for some period of time

**Describe the bug**
The cache doesn't work after globally set time offset expiration

**To Reproduce**
1. Set debug out mode on
2. Wait  $global:CurrencyConvExchangeRateCahcheLifetime expired - 60 seconds
3. Perform several requests

**Result**
All requests are sent to the server

**Expected behavior**
Only the first request performed- other cached