reverseproxy: Fix active health check header canonicalization, refactor

[francislavoie]

Fix #5444

[francislavoie]

The key here is this chunk of code:

	// Canonicalize the header keys ahead of time, since
	// JSON unmarshaled headers may be incorrect
	cleaned := http.Header{}
	for key, hdrs := range a.Headers {
		for _, val := range hdrs {
			cleaned.Add(key, val)
		}
	}
	a.Headers = cleaned

I also moved the provisioning logic to better localize it in the healthchecks file, moving the provisioning closer to the struct.

[mholt]

I think this refactor is a good cleanup. Thank you for the patch!