Fix factors tests properly.

[timcdlucas]

https://developer.r-project.org/blosxom.cgi/R-devel/NEWS/2020/02/28#n2020-02-28

Check now passes stringsAsFactors = TRUE. So all the downloaded data gets characters instead of factors that made some tests fail.

So far I've just hacked things. https://github.com/malaria-atlas-project/malariaAtlas/pull/50

Either we want to wait until stable and dev agree, and then write sensible tests. Or change getPR and other functions to be explicit.