/api/routes intermittently returns null.

[spicykoala]

I have a case where upon multiple hits of "/api/routes", null is intermittently returned. I noticed this because we have a task setup to poll this endpoint to check for route changes, and it was coming back true at odd intervals - turns out the request was returning null intermittently which was presented as a change.

(This is testable in the browser/postman)

[magiconair]

Since the routing table is atomically swapped I'd check the fabio logs if the routing table was temporarily empty. null shouldn't be able to happen (but I need to check the code to be certain)

See: https://github.com/fabiolb/fabio/blob/master/admin/api/routes.go#L28-L70