title: Computed write not working for arrays
body: <p>I have total calculation input boxes that are both readable and writeable.</p>
  
  <pre><code>&lt;div data-bind="foreach:TotalCostQuantity"&gt;
       &lt;input data-bind:"value: Cost, valueUpdate:'afterkeydown'" /&gt;&lt;br /&gt;
       &lt;input data-bind:"value: Quantity, valueUpdate:'afterkeydown'" /&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>My view model looks like this:</p>
  
  <pre><code>self.TrancheCostQuantity = ko.computed({
          read: function () {
              //do some math here to calculate cost and quant totals
  
              return [{Cost:100, Quantity:10},{Cost:200, Quantity:20}];
          },
          write: function (newValue) {
              customformat(newValue);
          }
      }, self);
  </code></pre>
  
  <p>The cost and quantity are from a grid that can have a dynamic number of rows and columns. It is mapped using the ko.mapping plugin.</p>
  
  <p>I need the write to fire but it never does. The read works fine but I can't validate or custom format my writes.</p>