Describe the bug
If a markdown file contains a double dash '--' it can not be loaded and reports an encoding error.
File is UTF-8. Removing the double dash allows rendering of the page. Writing the double dash as \-\- also works.

To Reproduce

mkdir test
cd test
git init
cat > test.md << EOF
# Test

ab --ab
EOF
git add test.md
git commit -m "test"
gollum

go to `localhost:4567/test.md

Expected behavior
Should show rendered markdown page.

Environment Info

$ gollum --versions
Gollum 5.2.1
Running on: java with Ruby version 2.5.7
Using:
rjgit 5.7.0.2
gollum-rjgit_adapter 0.6
gollum-lib 5.1
Markdown rendering gem: kramdown
Other renderering gems:
none