Preserve factor ordering in ggscatmat

[schloerke]

Fixes #329

https://cran.r-project.org/doc/manuals/r-devel/NEWS.html

R now uses a stringsAsFactors = FALSE default, and hence by default no longer converts strings to factors in calls to data.frame() and read.table().

When the stat_density was being added to the plot, the stat_density data was not a factor. This caused the original data to become "un-factored" (which caused the alphabetical sorting).

Now the factor ordering is preserved.

cc @andrewheiss @dicook

[dicook]

@schloerke this isn't actually fixed. i'm going to see if i can work it out