Improve test coverage

Test coverage isn't ideal, we need to ensure it has sufficient levels at least for core functionality:

- [ ] `Maltrap.Core.Models`
- [ ] `Maltrap.Core.Converters`
- [ ] `Maltrap.Core.Exceptions`
- [ ] `Maltrap.Core.Http`
- [ ] `Maltrap.Core.Rest`
- [ ] `Maltrap.Core.Extensions`

### Predecessors:
- [ ] #50 

