Fixes where('column', null) generating incorrect sql in query builder.

Let's say you want to filter your users, based on a column that is either a string value, or `null` and you only want users where their `status` column is `1`

Currently you would need to do 

    $users = User::where('status', 1); 

    if(is_null($status))
    {
        $users = $users->whereNull('column')->get();
    } 
    else 
    {
        $users = $users->where('column', $value)->get();
    }

This pull would allow you to do 

    $users = User::where('status', 1)->where('column', $value)->get();

And have it generate the correct SQL (`WHERE column IS null`),

presently if `$value` was `null` it would generate `WHERE column = null` which is not valid sql.