support block for undefined rpc request

[kazubu]

This request contains a change for support block in general(undefined) RPC call.
It introduce the ability to use nested elements in RPC request.

Example for use

dev = Netconf::SSH.new(login)
dev.open

dev.rpc.update_certificate{|x|
  x.type 'DEVICE_CERT'
  x.send(:'subject-common-name', 'cn.example.net')
  x.send(:'external-interfaces'){
    x.send(:'external-interface', '<External Port>')
  }
  x.send(:'internal-interfaces'){
    x.send(:'internal-interface', '<Internal Port>')
  }
  x.send(:'management-interface', 'true')
}