`add_footnote()` with `notation = "none"` produces "^^" in markdown

[turbanisch]

add_footnote() seems to enclose symbols used for numbering the footnotes with ^ in order to make them superscripts. With notation = "none", however, there is no symbol and ^^ will be rendered in the markdown output.

I think this is around where the superscript is being defined: https://github.com/haozhu233/kableExtra/blob/4c93f1afc26efcd04fa729cb4a522d5f262bb6bd/R/add_footnote.R#L81

Reprex:

library(tidyverse)
library(kableExtra)
#> 
#> Attaching package: 'kableExtra'
#> The following object is masked from 'package:dplyr':
#> 
#>     group_rows

dt <- head(mtcars, 5)
kbl(dt, format = "pipe") %>% 
  add_footnote(label = "foo", notation = "none")

	mpg	cyl	disp	hp	drat	wt	qsec	vs	am	gear	carb
Mazda RX4	21.0	6	160	110	3.90	2.620	16.46	0	1	4	4
Mazda RX4 Wag	21.0	6	160	110	3.90	2.875	17.02	0	1	4	4
Datsun 710	22.8	4	108	93	3.85	2.320	18.61	1	1	4	1
Hornet 4 Drive	21.4	6	258	110	3.08	3.215	19.44	1	0	3	1
Hornet Sportabout	18.7	8	360	175	3.15	3.440	17.02	0	0	3	2

Note: ^^ foo

^{Created on 2021-11-16 by the reprex package (v2.0.1)}