Gaps in ExtensionArray test coverage
We need tests that assert

1. `fillna` raises when passed an array of the wrong length
2. `fillna` returns a copy when there are no missing value
3. `take` raises when passed an invalid object for `fill_value` (may be somewhat tricky to do in a base test).

These tests would go in https://github.com/pandas-dev/pandas/blob/master/pandas/tests/extension/base/methods.py and https://github.com/pandas-dev/pandas/blob/master/pandas/tests/extension/base/getitem.py