httpx supports IDNA 2008 compliant domains when the idnx gem is installed (idnx is a FFI-based gem which uses libidn2 for punycode lookups):
# in Gemfile
gem "idnx"
gem "httpx"
# or via `gem`
# gem install idnx httpx, then:
HTTPX.get("http://Bücher.ch") #=> converts to "xn--bcher-kva.ch"
Next: DNS Resolvers