Bug 36649: Correctly embed biblio when retrieving items when adding to a train

I didn't find when this regression has been introduced but the third
parameter here is the headers, not parameters, we should not repeat
"headers".

Test plan:
1. Add a new processing that contains columns from the database, for example biblio.title and biblio.author.
2. Create a new train.
3. Add items to the waiting list.
4. Click on "Add last x items to the train" button.
5. Choose the train you created on number 2.
6. Choose the processing you added on number 1.
7. Try to click the Submit button.
=> Without the patch nothing happens and there is a JS error in the
console: Uncaught TypeError: item.biblio is undefined
=> With this patch applied the item is correctly added to the train and
the attribute is properly populated.

Signed-off-by: Anneli Österman <anneli.osterman@koha-suomi.fi>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 63612311e3)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

koha-tmpl/intranet-tmpl/prog/js/vue/components/Preservation/TrainsFormAddItems.vue

@@ -167,14 +167,12 @@ export default {
         async getItems(item_ids) {
             const client = APIClient.item
             let q = { "me.item_id": item_ids }
-            await client.items
-                .getAll(q, {}, { headers: { "x-koha-embed": "biblio" } })
-                .then(
-                    items => {
-                        this.items = items
-                    },
-                    error => {}
-                )
+            await client.items.getAll(q, {}, { "x-koha-embed": "biblio" }).then(
+                items => {
+                    this.items = items
+                },
+                error => {}
+            )
         },
         columnApiMapping(item, db_column) {
             let table_col = db_column.split(".")