Method new on collection does not work

Example : 
You have a Post model which has many Comment. 

You cannot do in reactive-record
`post.comments.new(title: "hello world")`
So you have to do 
`post.comments << Comment.new(title: "hello world")` 

This patch seem to fix the problem : 

```
module ReactiveRecord
  class Collection
    def new(*args)
      self << klass.new(*args)
    end
  end
end
```
