hostprototype.update API method allows to change inventory for templated host prototypes

Steps to reproduce:

1. Use a frontend to create templated host prototype;
2. Observe that frontend doesn't allow to update inventory mode once;
3. Use API client to update inventory mode:

   {
       "hostid": "10110",
       "inventory": {
           "inventory_mode": 0
       }
   }

Result:

Changes are made successfully.