Add failing test.

The test demonstrates failure for the following scenario. A single-subscription
client calls unsubscribe immediately followed by a subscribe. It will fail when
it tries to receive the next pmessage/message because the client will be in
false pub_sub_mode. Here is why it is false: First, the 2nd subscribe sets
pub_sub_mode to true during send_command. Next, the unsubscribe's
return_reply sets pub_sub_mode to false. The 2nd subscribe's return_reply does
not re-set pub_sub_mode back to true. So the result is a client with false
pub_sub_mode that fails upon receipt of the next message or pmessage.

tests/re_sub_test.js

@@ -0,0 +1,14 @@
+var client = require('../index').createClient()
+  , client2 = require('../index').createClient()
+  , assert = require('assert');
+
+client.once('subscribe', function (channel, count) {
+  client.unsubscribe('x');
+  client.subscribe('x', function () {
+    client.quit();
+    client2.quit();
+  });
+  client2.publish('x', 'hi');
+});
+
+client.subscribe('x');