Cannot pass Time parameter to time_entries.updated_since

This works:
```
h = Harvesting::Client.new
h.time_entries(updated_since: Date.yesterday)
```

This does not work:
```
h = Harvesting::Client.new
h.time_entries(updated_since: 1.hour.ago)
```

All attempts to pass time in addition to a date yield the following error:
**NoMethodError (undefined method `map' for nil:NilClass)**

I have tried with Time and DateTime classes, as well as passing a string (e.g. Time.now.iso8601) - all return this error, even though the Harvest API says time is acceptable in this field. I looked through time_entries.rb and see where this gem is trying to do a .map but got lost from there on where this is failing. Thanks in advance.