Make fwf_positions() guess end argument from start

[rcragun]

I would expect fwf_positions() to guess the value of end based on the value of start. If end is not specified, the best guess should be that end is lead(start)-1. Perhaps I misunderstand what arg=NULL means, but having end=NULL implies to me that end is optional. I believe that this function could be improved with some simple changes:

fwf_positions <- function (start, end, col_names = NULL) {
  if(missing(end)){
    end <- lead(start)-1L
  }
  col_names <- fwf_col_names(col_names, length(start))
  tibble(begin = start - 1L, end = end, col_names = col_names)
}

[jimhester]

end = NULL looks like it was inadvertantly added in #616, it should probably just be removed.

[jimhester]

Fixed by da99e55