TST, ENH: tests should set stringsAsFactors?

[drammock]

It is common for individual researchers to set stringsAsFactors=FALSE in their .Rprofile, so when running tests that expect dataframe columns to be factors, it would be better to do one of the following:

1. explicitly set options(stringsAsFactors=FALSE) (or TRUE) at the start of the test file, and restore its original value at end.

2. when comparing columns, use as.factor or as.character, as appropriate