Bug: updating a content library discards all draft docs in content search index

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Steps to reproduce:

1. Create a library in the Authoring MFE
2. Add a component to it and do not publish.
3. Edit the library's title (from the sidebar)
4. Note that the added component has disappeared from the components list

Expected behavior:

The only time we'd expect draft documents to be deleted is after calling "Discard changes" on a library with unpublished changes.

1. Create a library in the Authoring MFE
2. Add a component to it and do not publish.
3. Edit the library's title (from the sidebar)
4. The new component should remain in the components list.

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Closing this issue as I'm unable to reproduce it post-sumac cut.. so it's either been fixed or I was dreaming when I wrote this :)