Use form._meta instead of form.meta internally for backwards compatibility

Before WTForms 2 it was valid to have a form field called "meta" (I have such an example in a production system).

```python
import wtforms
class Form(wtforms.Form):
    meta = wtforms.StringField()
Form()
```
Now this will throw `AttributeError: 'StringField' object has no attribute 'wrap_formdata'`.

This patch should fix that, but continue to allow `form.meta` to be used for newer projects.