After render, single quote `'` is transformed to `‘’`

When I use rmarkdown::render an Rmd document, the quote changed between the Rmd and output md document.

The input text of test-encoding.Rmd

---
output: github_document
---

'test-rmarkdown'

But the output of test-encoding.md


‘test-rmarkdown’

I don't know why 'test-rmarkdown' is changed to ‘test-rmarkdown’.
Maybe is it related to my setting?

Here is my session information.

"si"
#> [1] "si"

Created on 2020-02-07 by the reprex package (v0.3.0)

Session info
devtools::session_info()
#> - Session info ---------------------------------------------------------------
#>  setting  value                                              
#>  version  R version 3.6.0 (2019-04-26)                       
#>  os       Windows 7 x64 SP 1                                 
#>  system   x86_64, mingw32                                    
#>  ui       RTerm                                              
#>  language (EN)                                               
#>  collate  Chinese (Simplified)_People's Republic of China.936
#>  ctype    Chinese (Simplified)_People's Republic of China.936
#>  tz       Asia/Taipei                                        
#>  date     2020-02-07                                         
#> 
#> - Packages -------------------------------------------------------------------
#>  package     * version    date       lib source                        
#>  assertthat    0.2.1      2019-03-21 [1] CRAN (R 3.6.0)                
#>  backports     1.1.5      2019-10-02 [1] CRAN (R 3.6.1)                
#>  callr         3.2.0      2019-03-15 [1] CRAN (R 3.6.0)                
#>  cli           2.0.1      2020-01-08 [1] CRAN (R 3.6.2)                
#>  crayon        1.3.4      2017-09-16 [1] CRAN (R 3.6.0)                
#>  desc          1.2.0      2018-05-01 [1] CRAN (R 3.6.0)                
#>  devtools      2.1.0      2019-07-06 [1] CRAN (R 3.6.1)                
#>  digest        0.6.19     2019-05-20 [1] CRAN (R 3.6.0)                
#>  evaluate      0.14       2019-05-28 [1] CRAN (R 3.6.0)                
#>  fansi         0.4.0      2018-10-05 [1] CRAN (R 3.6.0)                
#>  fs            1.3.1      2019-05-06 [1] CRAN (R 3.6.0)                
#>  glue          1.3.1      2019-03-12 [1] CRAN (R 3.6.0)                
#>  highr         0.8        2019-03-20 [1] CRAN (R 3.6.0)                
#>  htmltools     0.4.0      2019-10-04 [1] CRAN (R 3.6.1)                
#>  knitr         1.27       2020-01-16 [1] CRAN (R 3.6.2)                
#>  magrittr      1.5        2014-11-22 [1] CRAN (R 3.6.1)                
#>  memoise       1.1.0      2017-04-21 [1] CRAN (R 3.6.1)                
#>  pkgbuild      1.0.3      2019-03-20 [1] CRAN (R 3.6.1)                
#>  pkgload       1.0.2      2018-10-29 [1] CRAN (R 3.6.1)                
#>  prettyunits   1.0.2      2015-07-13 [1] CRAN (R 3.6.0)                
#>  processx      3.3.1      2019-05-08 [1] CRAN (R 3.6.0)                
#>  ps            1.3.0      2018-12-21 [1] CRAN (R 3.6.0)                
#>  R6            2.4.1      2019-11-12 [1] CRAN (R 3.6.2)                
#>  Rcpp          1.0.3      2019-11-08 [1] CRAN (R 3.6.2)                
#>  remotes       2.1.0      2019-06-24 [1] CRAN (R 3.6.1)                
#>  rlang         0.4.4      2020-01-28 [1] CRAN (R 3.6.0)                
#>  rmarkdown     2.1        2020-01-20 [1] CRAN (R 3.6.2)                
#>  rprojroot     1.3-2      2018-01-03 [1] CRAN (R 3.6.0)                
#>  sessioninfo   1.1.1      2018-11-05 [1] CRAN (R 3.6.0)                
#>  stringi       1.4.3      2019-03-12 [1] CRAN (R 3.6.0)                
#>  stringr       1.4.0      2019-02-10 [1] CRAN (R 3.6.0)                
#>  testthat      2.1.1      2019-04-23 [1] CRAN (R 3.6.1)                
#>  usethis       1.5.1.9000 2020-02-04 [1] Github (r-lib/usethis@e7c1f17)
#>  withr         2.1.2      2018-03-15 [1] CRAN (R 3.6.0)                
#>  xfun          0.8        2019-06-25 [1] CRAN (R 3.6.1)                
#>  yaml          2.2.0      2018-07-25 [1] CRAN (R 3.6.0)                
#> 
#> [1] C:/Program Files/R/R-3.6.0/library