Added ordering for dplyr locale issue.

[mbannick]

In version 1.1.0 of dplyr, they switched to ordering group_by by the system locale (e.g., "English") to "C". This means that something like c("a", "b", "A", "B") would be reordered to c("A", "B", "a", "b"). This caused issues internally and produced incorrect results, when someone did not use a constant case (like "a" or "A") in their treatment variable naming.

The implemented fix uses the already factored variable, rather than a version of the variable that's not a factor. So group_by does not have to rely on a locale to figure out the ordering.

The tests for this have to intentionally set the locale to be English. This is because the C locale is automatically set for the testthat suite, meaning the test could pass, but the undesirable behavior could still occur when it's run outside of the testing suite.