Fix resolution undefined

[sebastienbarbier]

Was easy to fix, reading code showed a case where conflict never initialised resolution object.
Fix #846

[vladikoff]

@sebastienbarbier

SUMMARY:
✔ 10 tests completed
⚠ 3 tests slow
✖ 1 test failed
FAILED TESTS:
  Authorization
    syncKinto
      ✖ should handle a conflict (slow: 1.394 secs)
        Firefox 59.0.0 (Linux 0.0.0)
      Data provided to an operation does not meet requirements.
      update@src/vendor/kinto.js:5941:7
      resolve/<@src/vendor/kinto.js:7144:7
      @src/vendor/kinto.js:5699:22
      in@src/vendor/kinto.js:5428:14
      execute/</<@src/vendor/kinto.js:5688:52
      $Promise@src/vendor/kinto-http.js:596:3141
      execute/<@src/vendor/kinto.js:5683:14
      step@src/vendor/kinto.js:5402:183
      step/<@src/vendor/kinto.js:5402:361
      s@src/vendor/kinto-http.js:596:1731
      notify/<@src/vendor/kinto-http.js:596:1870
      s@src/vendor/kinto-http.js:193:343

[sebastienbarbier]

Having those tests is awesome! will fix should not be much, thanks