ROBUSTNESS: Add explicit 'stringsAsFactors' arguments [cbind, rbind]

[henrikbengtsson]

ROBUSTNESS: Add explicit 'stringsAsFactors' arguments [cbind, rbind]

[HenrikBengtsson]

stringsAsFactors() has no effect here, e.g.

> str(rbind(data.frame(), list(a=4:5, b=letters[4:5])))
'data.frame':	2 obs. of  2 variables:
 $ a: int  4 5
 $ b: chr  "d" "e"

and

> data <- data.frame(a=1:3, b=letters[1:3], stringsAsFactors=F)
> str(rbind(data, list(a=4:5, b=letters[4:5])))
'data.frame':	5 obs. of  2 variables:
 $ a: int  1 2 3 4 5
 $ b: chr  "a" "b" "c" "d" ...