Django-graphene-auth

- Bug: when User.USERNAME_FIELD is set to `email`, login by secondary email was unsuccessful.
Fixed method `resolve_mutation` of the `ObtainJSONWebTokenMixin` class inside the `mixins` module.

- Added CommonTestCase class for testing both GraphQL and Relay queries.
- Cleanup testing code

- Fixed bug: when trying to refresh token (`mutation` `refreshToken`) with invalid refresh_token,
the response should contain null values for `refreshExpiresIn` output fields,
but the exception was thrown.

Changed property `required` to False for the fields `refreshExpiresIn` for
classes `RefreshToken` and `RelayRefreshToken`

- Fixed bug: when trying to login (`mutation` `tokenAuth`) with wrong credentials, the response should contain
null values for `payload` and `refreshExpiresIn` output fields, but the exception was thrown.

Changed property `required` to False for the fields `payload` and `refreshExpiresIn`

- Cleanup code