[R] RFE: vectorized behavior for as.POSIXct tz argument

[Jack Tanner]

x <- 1472562988 + 1:10; tz <- rep("EST",10)

# Case 1: Works as documented
ct <- as.POSIXct(x, tz=tz[1], origin="1960-01-01")

# Case 2: Fails
ct <- as.POSIXct(x, tz=tz, origin="1960-01-01")

If case 2 worked, it'd be a little easier to process paired (time, time zone)
vectors from different time zones.