frontend: use correct auto_prune default when creating via API
    
    Fix #1747
    Fix #1746
    
    The `test_update_copr_api3` test used web UI to create a new
    project. I changed it to API, which revealed that
    `follow_fedora_branching` and `runtime_dependencies` values weren't
    set at all, so I fixed it and updated the test.
    
    The behavior of `default` form values is terribly confusing to me, I
    described my observations in the `set_defaults` function. It might
    be wrong though.
    
    Also, please note that the #1747 issue cannot be reproduced via
    
        copr-cli create test-1 --chroot fedora-33-x86_64
    
    and neither it can't be reproduced via
    
        from copr.v3 import Client
        Client.create_from_config_file()
        client.project_proxy.add("frostyx", "test-2", ["fedora-33-x86_64"])
    
    because both of them send a complete `CoprForm` data. It can be
    reproduced only the way this test works, i.e. sending a subset of
    `CoprForm` to `/api_3/project/add/<NAME>`.