Don't raise when writing an attribute with and out of range time

The following code will raise an ``ArgumentError`` because of an out of range time:

````ruby
Model.new(:created_at => '2010-33-22T09:30:25Z')
````

However, this won't raise, this will set ``created_at`` to nil instead :

````ruby
Model.new(:created_at => 'Foobar')
````

I think the first example should act as the second one, hence this PR.

What do you think ?