[FIX] get interest for currency without interest

- `get_interest_at` for a currency without interest would not hit the intended check since the function was used as variable instead of calling the method
[FIX] Currency constructor with Currency object

clone the given Currency instance in `parse_json`
[TEST] add missing testcases for interest bearing currencies

- `get_interest_at` by providing a Date object
- `get_interest_at` for a Currency without interest