Issue Summary

When user update an External Import's configuration via Asset UI, then submit a PUT or PATCH to update the mapping via External Import API, it will wipe out the previous config changes via UI

Steps to Reproduce

1. Create External Import and submit initial mapping via External Import API
2. Update mapping on Asset UI such as changing behaviour with Missing Object etc
3. Submit a subsequent PATCH or PUT request via External Import API

Expected Results

Changes from step 2 above retained and any changes in step 3 merged with step 2

Actual Results

Everything is wiped and everything is replaced with Step 3

Workaround

Currently the only workaround is to ensure any changes required on Asset UI to be done as a final step before starting an import and not to submit any subsequent PATCH/PUT to mapping via External Import API afterward