split.xts should only call endpoints if length(f) == 1

[joshuaulrich]

split.xts calls endpoints(x, on = f, k = k) if is.character(f) is true. But that results in an error if f has length > 1.

require(xts)
data(sample_matrix)
x <- as.xts(sample_matrix)
split(x, format(index(x), "%m"))
Error in endpoints(x, on = f, k = k) : unsupported "on" argument

split.zoo should be dispatched if f is character and has length > 1.