Add unit test for callback on early postgresql disconnect

Test adapted by that provided by Jess Sheneberger in #534

test/unit/client/callback-on-early-disconnect.js

@@ -0,0 +1,22 @@
+var helper = require(__dirname + '/test-helper');
+var net = require('net');
+var pg = require('../../..//lib/index.js');
+ 
+var server = net.createServer(function(c) {
+  console.log('server connected');
+  c.destroy();
+  console.log('server socket destroyed.');
+  server.close(function() { console.log('server closed'); });
+});
+
+server.listen(7777, function() {
+  console.log('server listening');
+  var client = new pg.Client('postgres://localhost:7777');
+  console.log('client connecting');
+  client.connect(assert.calls(function(err) {
+    if (err) console.log("Error on connect: "+err);
+    else console.log('client connected');
+    assert(err);
+  }));
+   
+});