change default s3 method of as_md_tbl

#' @method as_md_tbl md_tbl

#' @rdname as_md_tbl

as_md_tbl.default <- function(x, ...) {

  x <- tryCatch(as.matrix(x), error = function(e) {

  msg <- paste("Cannot convert a", class(x)[1L], "object to md_tbl.")

  stop(msg, call. = FALSE)

  })

  as_md_tbl(x, ...)

}