add check prevening ownables from getting stuck

contracts/Ownable.sol

@@ -17,7 +17,7 @@ contract Ownable {
   }
 
   function transfer(address newOwner) onlyOwner {
-    owner = newOwner;
+    if (newOwner != address(0)) owner = newOwner;
   }
 
 }

test/Ownable.js

@@ -34,4 +34,20 @@ contract('Ownable', function(accounts) {
       .then(done)
   });
 
+  it("should guard ownership against stuck state" ,function(done) {
+    var ownable = Ownable.deployed();
+
+    return ownable.owner()
+      .then(function (originalOwner) {
+        return ownable.transfer(null, {from: originalOwner})
+          .then(function() {
+            return ownable.owner();
+          })
+          .then(function(newOwner) {
+            assert.equal(originalOwner, newOwner);
+          })
+          .then(done);
+      });
+  });
+
 });