udp: remove a needless instanceof Buffer check

When a string is passed to udpsock.send, it is automatically
converted to a Buffer. In that case, it is no longer needed
to test whether or not the argument is a Buffer or not.

PR-URL: nodejs#4301
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Reiss <me@silverwind.io>

lib/dgram.js

@@ -252,8 +252,7 @@ Socket.prototype.send = function(buffer,
 
   if (typeof buffer === 'string')
     buffer = new Buffer(buffer);
-
-  if (!(buffer instanceof Buffer))
+  else if (!(buffer instanceof Buffer))
     throw new TypeError('First argument must be a buffer or string');
 
   offset = offset | 0;