middleware/proxy: fix except keyword (#505)

Fix the except keyword usage - the config would allow it, but it was
not enforced in the code.
Turns out that **FROM** was also not enforced, fix both, by (basically)
copying the code from Caddy.

Update the README and tests.

Locally test as well, shows that this works:

~~~
.:1053 {
    proxy miek.nl 8.8.8.8:53 {
        except a.miek.nl
    }
    proxy a.miek.nl 8.8.4.4:53

    errors stdout
    log stdout
}
~~~

And gives the desired results, not having a proxy line for `a.miek.nl`
results in a SERVFAIL (as expected).

Fixes #502

middleware/proxy/upstream.go

@@ -46,7 +46,7 @@ func NewStaticUpstreams(c *caddyfile.Dispenser) ([]Upstream, error) {
 	var upstreams []Upstream
 	for c.Next() {
 		upstream := &staticUpstream{
-			from:        "",
+			from:        ".",
 			Hosts:       nil,
 			Policy:      &Random{},
 			Spray:       nil,
@@ -280,12 +280,13 @@ func (u *staticUpstream) Select() *UpstreamHost {
 	return u.Spray.Select(pool)
 }
 
-func (u *staticUpstream) IsAllowedPath(name string) bool {
+func (u *staticUpstream) IsAllowedDomain(name string) bool {
 	for _, ignoredSubDomain := range u.IgnoredSubDomains {
 		if dns.Name(name) == dns.Name(u.From()) {
 			return true
 		}
-		if middleware.Name(name).Matches(ignoredSubDomain + u.From()) {
+
+		if middleware.Name(ignoredSubDomain).Matches(name) {
 			return false
 		}
 	}