Improve test coverage

[lucafuji]

Describe the bug We previously had a test coverage of 82% but now it drops to 75%. Ideally, code coverage should be > 80% for a healthy repo.

To Reproduce Steps to reproduce the behavior:

1. go to https://codecov.io/gh/uber/aresdb you will see detailed overage for each package, file, method, and lines.

Expected behavior Ideally, code coverage should be > 80% for a healthy repo.

Screenshots