Call to IsSame in TestDuplicateBootstrap fails

[zenhack]

This is a loose end from #515, where we decided to merge without dealing with this failure:

$ go test
--- FAIL: TestDuplicateBootstrap (0.12s)
    level0_test.go:1391: 
        	Error Trace:	/home/isd/src/go-wk/go-capnp/rpc/level0_test.go:1391
        	Error:      	Should be true
        	Test:       	TestDuplicateBootstrap
    level0_test.go:2254: conn error: rpc: remote abort: connection closed
FAIL
exit status 1
FAIL	capnproto.org/go/capnp/v3/rpc	0.491s