rpc: intermittent test failure

It seems it happens rarely (seen once so far - subsequent runs of the tests pass). Seen at r947, but probably was there before that.

FAIL: rpc_test.go:408: suite.TestBadCall

[LOG] 37.26191 JUJU rpc: discarding obtainer method reflect.Method{Name:"callError", PkgPath:"launchpad.net/juju-core/rpc_test", Type:(*reflect.commonType)(0x6ce868), Func:reflect.Value{typ:(*reflect.commonType)(0x6ce868), val:(unsafe.Pointer)(0x4259ba), flag:0x131}, Index:3}
[LOG] 37.26196 JUJU rpc: discarding obtainer method reflect.Method{Name:"called", PkgPath:"launchpad.net/juju-core/rpc_test", Type:(*reflect.commonType)(0x6b0638), Func:reflect.Value{typ:(*reflect.commonType)(0x6b0638), val:(unsafe.Pointer)(0x425d00), flag:0x131}, Index:4}
[LOG] 37.26200 JUJU rpc: discarding obtainer method reflect.Method{Name:"testCall", PkgPath:"launchpad.net/juju-core/rpc_test", Type:(*reflect.commonType)(0x6e9858), Func:reflect.Value{typ:(*reflect.commonType)(0x6e9858), val:(unsafe.Pointer)(0x426c18), flag:0x131}, Index:5}
[LOG] 37.26253 JUJU send client request header: &rpc.Request{RequestId:0x1, Type:"BadSomething", Id:"a0", Request:"No"}
[LOG] 37.26272 JUJU send client request body: struct {}{}
[LOG] 37.26278 JUJU got response header &rpc.Response{RequestId:0x0, Error:"", ErrorCode:""}
rpc_test.go:417:
[LOG] 37.26292 JUJU rpc: client protocol error: read tcp [::1]:35405: connection reset by peer
    c.Assert(err, ErrorMatches, `server error: unknown object type "BadSomething"`)
... error string = "write tcp [::1]:35405: broken pipe"
... regex string = "server error: unknown object type \"BadSomething\""

OOPS: 8 passed, 1 FAILED
--- FAIL: TestAll (0.06 seconds)