Use hyper-reverse-proxy fork with fixed host header behavior:

Cargo.lock

@@ -1610,7 +1610,7 @@ dependencies = [
 [[package]]
 name = "hyper-reverse-proxy"
 version = "0.5.2-dev"
-source = "git+https://github.com/felipenoris/hyper-reverse-proxy.git?rev=e73a76600ce9e51e962de5266b03be596e6c1d50#e73a76600ce9e51e962de5266b03be596e6c1d50"
+source = "git+https://code.betamike.com/micropelago/hyper-reverse-proxy.git?branch=dont-set-host-header#9f4b94724f9b164d2e2d08607780d5e85f53368e"
 dependencies = [
  "hyper",
  "lazy_static",

Cargo.toml

@@ -52,4 +52,4 @@ hyper-trust-dns = "0.5.0"
 
 # The micropelago fork of tokio-rustls allows for gemini proxying
 tokio-rustls = { git = "https://code.betamike.com/micropelago/tokio-rustls.git", branch = "start-handshake-into-inner" }
-hyper-reverse-proxy = { git = "https://github.com/felipenoris/hyper-reverse-proxy.git", rev = "e73a76600ce9e51e962de5266b03be596e6c1d50" }
+hyper-reverse-proxy = { git = "https://code.betamike.com/micropelago/hyper-reverse-proxy.git", branch = "dont-set-host-header" }