cop_auth: host domain should respond to ping

[jphines]

Problem

We identified a strange bug in our routing logic in which the /ping endpoint was getting a 404 if and only if we used the configured host in the request. So any requests that used any other host responded with a 200 🐛.

This is a result of the way we've set up our mux in combination with go's default mux handling. Patterns that match a host domains take precedent, so when the mux receives the request with the host we've configured, it walks down the mux subtree that matched with the host and 404's since we haven't set up a handler for /ping in that mux subtree.

However, if we use a host that is not configured, the pattern does not match on the host domain and falls into our catchall which we've configured to respond to /ping requests.

Solution

We configure /ping on the configured host patterns also.

Notes

So /ping should now respond to all domains, regardless of what the host was passed in the request.

cc @pyoio