When changing item prototype type from HTTP agent to SSH agent, username and password fields not being saved.

Steps to reproduce:

1. Create an item prototype of type HTTP agent.
2. Reopen the item prototype configuration.
3. Change the type to SSH agent.
4. Fill Username and Password fields.
5. Save the changes.
6. Reopen the item prototype configuration again.

Result:
Username and Password fields are blank.

Expected:
Username and Password fields are filled.

When updating item prototype type from SSH agent to HTTP agent, username and password fields not being reset.

Steps to reproduce:

1. Create an item prototype of type SSH agent.
2. Fill Username and Password fields.
3. Reopen the item prototype configuration.
4. Change the type to HTTP agent.
5. Save the changes.
6. Reopen the item prototype configuration again and change the type back to SSH agent.

Result:
Username and Password fields are filled.

Expected:
Username and Password fields are blank.