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

[stadub]

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