Improve unit tests for pandas_helpers

[MahmoodEtedadi]

Is your feature request related to a problem? Please describe

Given that subtle changes in pandas behavior could impact how event windows are handled, it would be prudent to add more comprehensive unit tests for the relevant functions.

Describe the solution you'd like

Adding more unit tests for pandas_helpers (in particular, merge_windowed_event) would be helpful.

[gbowlin]

While #155 added a fair number of tests to pandas helpers, we still want to improve the quality of unit tests.

In particular - we should try to come up with minimal data frame examples for each test that are easy to validate by hand. This way we can be sure that tests only cover the data they need to run, and that the example makes sense given the name of the test.