README.md not changed to index.html in files other than summary

[nihaals]

docker run -it --rm rust:slim bash
# cargo install mdbook
# mkdir /app
# cd /app
# mdbook init
...
# printf "[Test](./test/README.md)\n[Test 2](./test.md)" > src/SUMMARY.md
# printf "[Test](./test/README.md)" > src/test.md
# mkdir src/test
# printf "# Test" > src/test/README.md
# mdbook build
# cat book/test.html | grep README.html
<p><a href="./test/README.html">Test</a></p>
# ls book/test
index.html

[ehuss]

I believe this is a duplicate of both #984 and #1268, so closing in favor of those.