title: angularjs: expression is non-assignable
body: <p>I'm using <code>ui-gmap-google-map</code>, where I'm defining coordinates on the  like:</p>
  
  <pre><code>&lt;div ui-gmap-google-map my-google-maps center='{  latitude: 51.4994961, longitude: -0.1246853 }' zoom="12"&gt;
  </code></pre>
  
  <p>This throws the error:</p>
  
  <pre><code>Expression '{  latitude: 51.4994961, longitude: -0.1246853 }' used with directive 'uiGmapGoogleMap' is non-assignable!
  </code></pre>
  
  <p>Someone else's reported the same issue and the apparent solution's here <code>https://github.com/angular-ui/angular-google-maps/issues/418</code>, but how to use ng-init to define an object for a scope property that is used to initialise a directive ? From what I know, I have to filter the string to json parse before setting the scope.center ? </p>