fix(lib): enable multiple consecutive slash support

Closes #754
Closes #808
Closes #815

Apparently more applications use multiple slashes in a row than I
thought. There is no easy way around this other than to do this hacky
fix to avoid net/http#ServeMux's URL cleaning.

test/double_slash/test.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+function cleanup() {
+	pkill -P $$
+}
+
+trap cleanup EXIT SIGINT
+
+# Build static assets
+(cd ../.. && npm ci && npm run assets)
+
+go tool anubis --help 2>/dev/null || :
+
+go run ../cmd/httpdebug &
+
+go tool anubis \
+	--policy-fname ./anubis.yaml \
+	--use-remote-address \
+	--target=http://localhost:3923 &
+
+backoff-retry node ./test.mjs