title: using ...spread, but redux still throws warning about state mutation
body: <p>Redux throws Warning on dispatch:</p>
  
  <pre><code>Error: A state mutation was detected inside a dispatch, in the path: 
  roundHistory.2.tickets. Take a look at the reducer(s) handling the action {"type":"ARCHIVE_TICKETS","roundId":"575acd8373e574282ef18717","data":[{"_id":"575acd9573e574282ef18718","value":7,"user_id":"574c72156f355fc723ecdbbf","round_id":"575acd8373e574282ef18717","__v":0},{"_id":"575acd9573e574282ef18719","value":9,"user_id":"574c72156f355fc723ecdbbf","round_id":"575acd8373e574282ef18717","__v":0},{"_id":"575acd9573e574282ef1871a","value":8,"user_id":"574c72156f355fc723ecdbbf","round_id":"575acd8373e574282ef18717","__v":0},{"_id":"575acdac73e574282ef1871b","value":19,"user_id":"574c72156f355fc723ecdbbf","round_id":"575acd8373e574282ef18717","__v":0},{"_id":"575ad50c4c17851c12a3ec23","value":29,"user_id":"57522f0b1f08fc4257b9cbc6","round_id":"575acd8373e574282ef18717","__v":0},{"_id":"575ad50c4c17851c12a3ec24","value":28,"user_id":"57522f0b1f08fc4257b9cbc6","round_id":"575acd8373e574282ef18717","__v":0},{"_id":"575ad
  </code></pre>
  
  <p>The only reducer handling <code>ARCHIVE_TICKETS</code> action is this one:</p>
  
  <pre><code>case 'ARCHIVE_TICKETS' :
    var archive = [...state.roundHistory];
    for (var i in archive) {
      if (archive[i]._id === action.roundId) {
        archive[i].tickets = action.data;
      }
    }
    return Object.assign({}, state, {
      roundHistory: archive
    });
  </code></pre>
  
  <p>How can it mutate the state if i use [...spread]?</p>