couchrest models don't save if declared a property with name "properties", and fail without any exceptions

Given a class like this:

```
class MyProduct < Couchrest::Model::Base
  property :name
  property :price
  property :properties      <<<---- the culprit

  ### some more code here
end
```

when running this code:

```
prod = MyProduct.new(attrs)
prod.save
```

it doesn't raise any exceptions at all, and at the same time it doesn't create any couch documents.
