custom attribute erased when updating new reservation form

[WhilelM]

Describe the bug**

custom attribute erased when updating new reservation form

To Reproduce**

On a New Reservation form, with Title and Description fields fullfiled. And a custom attribute (Multiple Line Textbox) fullfiled too.
When modifying owner's reservation, custom attribute is cleaned when others fields stay unmodified.

Expected behavior**

I wish that the custom attribute stay unmodified too.

Screenshots**

before changing owner
after changing owner

Specifications

• LibreBooking version 4.0.0
• relevant with version 2.8.6.2