Update Consumer.php

```
$this->_use_assocs = (is_null($this->store) ? false : true);
```
always return true, even the store is Auth_OpenID_DumbStore, and do an association with the OpenID Server.

but OpenID Server will reject the check_authentication, if RP have do an association.
Merge pull request #111 from stutiredboy/master

Update Consumer.php for Auth_OpenID_DumbStore