Lookup public IP using HTTP

Both OpenDNS and Google DNS has problems looking up the public IP on
IPv6 connections. Lookup up using HTTP (TCP) seems more stable. The
lookup time is about 110ms longer with this mechanism.

checkip.amazonaws.com for now only resolvs to IPv4 addresses. If that
would change we would need to lookup the A address of the domain and
connect to that IP manually. It's like 2 more lines of code only, if
connect_timeout isn't needed, then it's a bit more involved.

lib/sparoid.rb

@@ -149,9 +149,16 @@ def write_cache
     end
   end
 
-  def public_ip
-    Resolv::DNS.open(nameserver: ["208.67.222.222", "208.67.220.220"]) do |dns|
-      dns.getresource("myip.opendns.com", Resolv::DNS::Resource::IN::A).address
+  def public_ip(host = "checkip.amazonaws.com")
+    Socket.tcp(host, 80, connect_timeout: 3) do |sock|
+      sock.sync = true
+      sock.print "GET / HTTP/1.1\r\nHost: #{host}\r\nConnection: close\r\n\r\n"
+      status = sock.readline(chomp: true)
+      raise ResolvError.new("#{host} response: #{status}") unless status.start_with? "HTTP/1.1 200 "
+      until sock.readline(chomp: true).empty? 
+        # skip all headers
+      end
+      sock.readline(chomp: true) # this is the IP
     end
   end