Describe the bug
When turning on --exportDefault it returns a default export for all files even if the file itself has no default export. This is a issue since only some of my files has default exports.

To Reproduce
Steps to reproduce the behavior:

1. create a project
2. Create some files and mix default exports and non defaults
3. run barrelsby with --exportDefault
4. All files in the index file gets a default export eventhough some does not have it

Expected behavior
only export default exports when they're available

Screenshots