Fix header casing for Rack 3

[mitchellhenke]

Follow up to #9359 where there was an issue with requests failing due to headers not being lowercase. Issue reported here.

I wasn't able to find a good way to test the public file server headers though.

[aduth]

I wasn't able to find a good way to test the public file server headers though.

I think this'll happen if you compile in a separate process from make run such that the typical Webpack development server isn't used.

e.g.

yarn build && yarn build:css
# or: rails assets:precompile
rails s

I just did this and confirmed the headers are now lowercase.

[charleyf]

This solved the problem for me.