apa_table: Added NA-column and [tbp] in appendix

[TobiasHeycke]

An additional column named NA. is added to table, spaces in column names are substituted by points and removing 'added_stub_head' does not work. See the following script:

my_table <- apply(cars, 2, function(x)
   c(Mean = mean(x), SD = sd(x), Min = min(x), Max = max(x))
)

#when column names include a space, a point will be added (no problem with rownames)
colnames(my_table)<- c("top speed", "max dist")


apa_table(my_table, added_stub_head = "Descriptives")

#Not using the added_stub_head argument adds the rownames in that field
apa_table(my_table)