Prevent child partial template usage from polluting original template data

When using a partial template from a child using this syntax:

``` ruby
child :object, :partial => 'child/template'
```

The orignal compiled template is polluted with the `_data` attribute from the first call to `child`. This prevents the template from being reused as the now stored on template `_data` method may not be accessible on other objects, causing method not found exceptions. This small change only adds the `_data` attribute to the copy of the template being used for rendering, preventing the template from becoming corrupted for reuse.