DNSServer: Handle examplewww.com correctly

Just replacing 'www.' with the empty string when we assign the
domainname will remove all occurrences of 'www.', not just those
at the start of the string.

Change this to a startsWith check so that only "www." at the
beginning of the string is removed.

libraries/DNSServer/src/DNSServer.cpp

@@ -46,7 +46,8 @@ void DNSServer::stop()
 void DNSServer::downcaseAndRemoveWwwPrefix(String &domainName)
 {
   domainName.toLowerCase();
-  domainName.replace("www.", "");
+  if (domainName.startsWith("www."))
+      domainName.remove(0, 4);
 }
 
 void DNSServer::processNextRequest()