Bug 28382: Set trigger to 'Reserve' when actioning a reserve transfer

Bug 28382: Set trigger to 'Reserve' when actioning a reserve transfer

This patch sets the transfer trigger to 'Reserve' when the staff user
confirms they are transfering to fulfil a reserve.

Test plan
1 - Place a hold for delivery at Library B
2 - Sign in at Library A
3 - Browse to Circulation->Transfers
4 - Attempt to transfer an item on the title with the hold to Library B
5 - You get a notice that hold was found
6 - You have the option transfer the for the hold - click it
7 - Confirm hold is set to transit and transfer generated
8 - Check the branchtransfers table and confirm the 'reason' is
  'Reserve' not 'Manual'.

Signed-off-by: David Nind <david@davidnind.com>
JK: fix typo resver -> reserve
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>