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/anubis.yaml

@@ -0,0 +1,8 @@
+bots:
+  - name: challenge
+    user_agent_regex: CHALLENGE
+    action: CHALLENGE
+
+status_codes:
+  CHALLENGE: 200
+  DENY: 403