title: calls to angular.extend don't cause $scope.$watch to proc
body: <p>I'm merging a smaller object into a larger object using angular.extend. </p>
  
  <p>Here's a sample of data:</p>
  
  <pre><code>$scope.bigDocument = {
      "id": 0,
      "guid": "e75ce3b3-68f6-423f-94d8-1a613cde0c59",
      "isActive": true,
      "balance": "$2,437.00",
      "picture": "http://placehold.it/32x32",
      "age": 38,
      "name": "Parks Williamson",
      "gender": "male",
      "company": "Arctiq",
      "email": "parkswilliamson@arctiq.com",
      "phone": "+1 (817) 488-3119",
      "address": "252 Concord Street, Morgandale, Arizona, 1866",
      "about": "Irure nostrud nisi qui do amet nisi adipisicing dolor eiusmod do non laboris.",
      "registered": "1995-04-08T20:16:47 +05:00",
      "latitude": -6.796341,
      "longitude": 40.321499,
      "randomArrayItem": "cherry"
  }
  
  $scope.littleDocument = {
       "balance": "$3,193.00",
       "picture": "http://placehold.it/32x32",
       "age": 22, 
       "name": "Coffey Wilcox",
       "gender": "male",
       "company": "Balooba",
       "email": "coffeywilcox@balooba.com",
       "phone": "+1 (824) 518-3639",
       "address": "836 Douglass Street, Imperial, Montana, 9365",
       "about": "Ipsum dolore officia consectetur proident occaecat.",
       "registered": "1994-12-12T15:47:09 +06:00",
       "latitude": -33.474425,
       "longitude": -113.998081,
  }
  </code></pre>
  
  <p>I'm using the following function to merge:</p>
  
  <pre><code>$scope.merge = function() {
      angular.extend($scope.bigDocument, $scope.littleDocument)
  </code></pre>
  
  <p>and watching for changed with the following</p>
  
  <pre><code>$scope.$watch('bigDocument', function(newVal, oldVal) {
      console.log('bigDocument Changed');
  })
  </code></pre>
  
  <p>I get the expected output on initial hydration of $scope.bigDocument, but not after the merge function is called. The data changes correctly, but the $watch method never gets hit. </p>
  
  <p>What gives?</p>
  
  <p>Here's a plunker (in coffeescript) to show the full workflow: <a href="http://plnkr.co/edit/VPnxhUa6uz1zKfcufb2J" rel="nofollow noreferrer">http://plnkr.co/edit/VPnxhUa6uz1zKfcufb2J</a></p>