Coerce matrix to data.frame in writexlsx.R

[grantbrown]

write.xlsx produces empty or incorrectly formatted files if the argument is a matrix instead of a data frame. This is one potential workaround for a (potentially) common mistake in usage. An alternative approach would be to more explicitly enforce the type of the x argument.

Simple example which will silently fail without something like the embedded commit:

library(openxlsx)
X <- matrix(rnorm(100), ncol = 10)
write.xlsx(X, file = "tmp.xlsx", replace = TRUE)