blockchain: get_account_reputations fix for first

`get_account_reputations` called with the `first` parameter set
was skipping the first result also for the first request in the
loop, effectively missing one result.

- only skip the first result for follow-up requests
- add test case for `get_account_reputations()`
- replace `steem_instance` with `blockchain_instance`
- add test case for `get_account_count()`