Fix a bug in fetching the XML URL.

In Discover, the reponse was lowercased for comparison.  However,
this caused a 404 - Not found when fetching the url provided by
the location header if the url contained uppercase.

ok @owainga

upnp.go

@@ -110,8 +110,7 @@ func Discover() (nat NAT, err error) {
 		// HTTP header field names are case-insensitive.
 		// http://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2
 		locString := "\r\nlocation: "
-		answer = strings.ToLower(answer)
-		locIndex := strings.Index(answer, locString)
+		locIndex := strings.Index(strings.ToLower(answer), locString)
 		if locIndex < 0 {
 			continue
 		}