Shiny-ggvis export to svg: encoding error

shiny
,
1

When a ggvis plot in shiny contains special characters, like the Umlaut ä, the plot looks good on screen. If I download the plot as SVG using the wheel in the upper right corner, the generated SVG-file is encoded using ISO-8859 instead of UTF-8:
TestPlot.svg: ISO-8859 text, with very long lines, with no line terminators.

This yields to an error if one tries to view it:
XML Parsing Error: not well-formed
Location: file:///home/thn/Downloads/TestPlot.svg
Line Number 1, Column 4855:

After changing the encoding to utf-8 (iconv -f ISO-8859-1 -t utf-8 ...), the SVG is displayed properly.

Here the R code:

library(ggvis)
library(shiny)

ui <- fluidPage(fluidRow(column(width=12,ggvisOutput(plot_id="TestPlot"))))

server <- function(input, output) {
  observe(x={
    data.frame(x=c(0,1,2,3,4),y=c(6,4,7,8,0)) %>% 
      ggvis(~x, ~y) %>% 
      layer_points() %>%
      add_axis(type="x",title="Umlaut ä") %>%
      bind_shiny("TestPlot")})
}

shinyApp(ui,server)

Can anybody indicate any help?
Would it be possible to pipe the SVG to iconv within shiny?