Existing account cannot be associated to external identiy

re: #13 , it does not seem possible to associate a local account to an external identity.

Plone 5.2.0
pas.plugins.authomatic 1.0b2

1. Create a new Plone site
2. Activate pas.plugins.authomatic with a valid JSON configuration for github
3. In `@@usergroup-userprefs`, add a new local user
4. In a new incognito window, use `failsafe_login_form` to log in as the local user that was just created
5. Go to `@@authomatic-handler` and click the Github button under *Add identity*

Result:

```
Traceback (innermost last):
Module ZPublisher.WSGIPublisher, line 155, in transaction_pubevents
Module ZPublisher.WSGIPublisher, line 337, in publish_module
Module ZPublisher.WSGIPublisher, line 255, in publish
Module ZPublisher.mapply, line 85, in mapply
Module ZPublisher.WSGIPublisher, line 61, in call_object
Module pas.plugins.authomatic.browser.view, line 115, in __call__
ValueError: Provider github is already connected to current user.
```