travis run: https://travis-ci.org/bitcoin/bitcoin/jobs/406813397

```
rpc_psbt.py failed, Duration: 4 s
stdout:
2018-07-22T13:28:57.765000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_test_runner_20180722_132530/rpc_psbt_49
2018-07-22T13:29:00.531000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/test_framework.py", line 160, in main
    self.run_test()
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/rpc_psbt.py", line 101, in run_test
    assert_raises_rpc_error(-22, "TX decode failed", self.nodes[0].converttopsbt, signedtx['hex'])
  File "/home/travis/build/bitcoin/bitcoin/build/bitcoin-x86_64-unknown-linux-gnu/test/functional/test_framework/util.py", line 105, in assert_raises_rpc_error
    assert try_rpc(code, message, fun, *args, **kwds), "No exception raised"
AssertionError: No exception raised
unknowns.pushKV(HexStr(entry.first), HexStr(entry.second));
            }
            out.pushKV("unknown", unknowns);
        }
        outputs.push_back(out);
        
        output_value += psbtx.tx->vout[i].nValue;
    }
    result.pushKV("outputs", outputs);
    if (have_all_utxos) {
        result.pushKV("fee", ValueFromAmount(total_in - output_value));
    }
    return result;
}
UniValue combinepsbt(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            RPCHelpMan{"combinepsbt",
                "\nCombine multiple partially signed Bitcoin transactions into one transaction.\n"
                "Implements the Combiner role.\n",
                {
                    {"txs", RPCArg::Type::ARR, RPCArg::Optional::NO, "A json array of base64 strings of partially signed transactions",
                        {
                            {"psbt", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "A base64 string of a PSBT"},
                        },
                        },
                },
                RPCResult{
            "  \"psbt\"          (string) The base64-encoded partially signed transaction\n"