You can't assign a string to a list, and I check for that now.

```
1) ERROR: xblock.test.test_runtime.test_db_model_keys

   Traceback (most recent call last):
    env/lib/python2.7/site-packages/nose/case.py line 197 in runTest
      self.test(*self.arg)
    xblock/test/test_runtime.py line 155 in test_db_model_keys
      setattr(tester, field.name, new_value)
    xblock/fields.py line 391 in __set__
      value = self.enforce_type(value)
    xblock/fields.py line 599 in from_json
      raise TypeError('Value stored in an List must be None or a list, found %s' % type(value))
   TypeError: Value stored in an List must be None or a list, found <type 'str'>
```

Changed the code to be more explicit about `List` and not specific to `tags`.