Do not try to idna convert IPv6 addresses

This will fail when the package idna is in use on Python 2. My guess
from a quick skim of the spec is that trying to apply IDNA to IP
adresses is wrong.

See also kjd/idna#58

eventlet/support/greendns.py

@@ -511,9 +511,9 @@ def getaddrinfo(host, port, family=0, socktype=0, proto=0, flags=0):
     flag ensures getaddrinfo(3) does not use the network itself and
     allows us to respect all the other arguments like the native OS.
     """
-    if isinstance(host, six.string_types):
-        host = host.encode('idna').decode('ascii')
     if host is not None and not is_ip_addr(host):
+        if isinstance(host, six.string_types):
+            host = host.encode('idna').decode('ascii')
         qname, addrs = _getaddrinfo_lookup(host, family, flags)
     else:
         qname = host