etcdhttp: check method for every endpoint, add tests

the previous one is good enough...
If we want to make it clear, probably this one?
``` go
// ms Must be comma separated. It is the caller's responsibility to 
// provide the right format.
func allowMethod(w http.ResponseWriter, m string, ms string) bool {
    if strings.Contains(ms, m) {
        return true
    }
    ...
    return false
}
```