Fix schemas setting for 'search' drawer item

Changing schemas of a search drawer item does not get saved.
How to reproduce:

• in Studio Designer, navigate to UI > Drawer > Drawer Items and create a new Search drawer item
  • Set its name
  • under section Element & Attributes select a page provider in field provider and add a schema e.g. dublincore in field Schemas (other than the one(s) automatically added)
• save the Search drawer item
• navigate away then navigate back to the Search drawer item you just created

Expected result: the schemas field contains the schema you added (dublincore)
Actual result: the schemas field does not contain the schema you added (dublincore)