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
{
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 2)
        throw std::runtime_error(
            RPCHelpMan{"finalizepsbt",
                "Finalize the inputs of a PSBT. If the transaction is fully signed, it will produce a\n"
                "network serialized transaction which can be broadcast with sendrawtransaction. Otherwise a PSBT will be\n"
                "created which has the final_scriptSig and final_scriptWitness fields filled for inputs that are complete.\n"
                "Implements the Finalizer and Extractor roles.\n",
                {
                    {"psbt", RPCArg::Type::STR, RPCArg::Optional::NO, "A base64 string of a PSBT"},
                    {"extract", RPCArg::Type::BOOL,  "true", "If true and the transaction is complete,\n"
            "                             extract and return the complete transaction in normal network serialization instead of the PSBT."},
                },
                RPCResult{
            "{\n"
            "  \"psbt\" : \"value\",          (string) The base64-encoded partially signed transaction if not extracted\n"
            "  \"hex\" : \"value\",           (string) The hex-encoded network transaction if extracted\n"
            "  \"complete\" : true|false,   (boolean) If the transaction has a complete set of signatures\n"
            "  ]\n"
            "}\n"
                },
                RPCExamples{
                    HelpExampleCli("finalizepsbt", "\"psbt\"")
                },
            }.ToString());
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValue::VBOOL}, true);
    
    PartiallySignedTransaction psbtx;
    std::string error;
    if (!DecodeBase64PSBT(psbtx, request.params[0].get_str(), error)) {