Skip filename encoding tests on non-UTF-8 Unix platforms

[wch]

Resolves #264.

To verify:

docker run --security-opt seccomp=unconfined --rm -ti rocker/r-devel /bin/bash

apt install -y git libssl-dev vim nano

echo en_US ISO-8859-1 >> /etc/locale.gen
locale-gen

echo MAKEFLAGS=-j4 > ~/.Renviron

git clone https://github.com/rstudio/httpuv.git
RD -e "install.packages('remotes')"
RD -e 'remotes::install_deps("httpuv", dependencies = TRUE)'

export LANG=en_US.iso88591
# This prints a warning, but it seems to be necessary for things to work.
export LC_ALL=en_US.iso88591

RD --quiet -e 'Sys.getlocale()'

# Get the right branch
cd httpuv
git checkout  wch-filename-encoding
cd ..

RD CMD build httpuv
R CMD check httpuv_1.5.2.9000.tar.gz 

This will pass, but if you check out the master branch, it will fail.