The second tabset does not work properly when the tab names are UTF-8 chinese characters

Hi,

The reproducible Rmd file is here: https://gist.github.com/wush978/d8213f78cdab4949dcef50a5b9eb6e9f

Note that the tabset becomes normal when I use English title.

Here is my sessionInfo while triggers this bug:

R version 3.4.1 (2017-06-30)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.2 LTS

Matrix products: default
BLAS: /usr/lib/libblas/libblas.so.3.6.0
LAPACK: /usr/lib/lapack/liblapack.so.3.6.0

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] shiny_1.0.5

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.13      digest_0.6.12     rprojroot_1.2    
 [4] mime_0.5          R6_2.2.2          jsonlite_1.5     
 [7] xtable_1.8-2      backports_1.1.1   magrittr_1.5     
[10] evaluate_0.10.1   stringi_1.1.5     rmarkdown_1.6    
[13] flexdashboard_0.5 tools_3.4.1       stringr_1.2.0    
[16] httpuv_1.3.5      yaml_2.1.14       compiler_3.4.1   
[19] htmltools_0.3.6   knitr_1.17       

Start with the reproducible Rmd file linked in the issue and compare the behavior of the second tabset with English and Chinese tab names. Reproduce it under the documented UTF-8 Ubuntu/R session, then trace the tabset handling to identify the encoding-dependent behavior. Done means both tabsets work correctly when tab names use UTF-8 Chinese characters.