Drush features-export should not fallback to default bundle when exporting an existing feature

Problem/Motivation

When using drush fex to export an existing feature, if you forget to provide the --bundle option, Features assumes you are using the default bundle even if the features.yml files or the feature name says otherwise.

In my case, the main issue is that my "Optional" assignment plugin is not configured like the default bundle's one so everytime I export my features via drush, all the files got messed up.

Proposed resolution

Use the bundle of the features we export instead of the default bundle.

Remaining tasks

Patch, review, commit.

User interface changes

None.

API changes

None.

Data model changes

None.