title: Why breeze sends to server originalvaluesMap for unmapped property?
body: <p>I'm using breeze 1.4.11 with Web Api and EFContextProvider</p>
  
  <p>on client after metadata fetched I extend entity type:</p>
  
  <pre><code>var addressStringProperty = new breeze.DataProperty({
      name: "addressString",
      isUnmapped: true
  })
  metadataStore.getEntityType('Account').addProperty(addressStringProperty);
  </code></pre>
  
  <p>this property is computed and used only on client</p>
  
  <p>after <code>entitymanager.saveChanges([accountEntity])</code> I see on server side in <code>contextprovider.BeforeSaveEntity</code> that <code>entityInfo.OriginalvaluesMap</code> contains Key "AddressString" with Value == null.</p>
  
  <p>same thing with extending entity like:</p>
  
  <pre><code>var accountCtor = function() {
    this.addressString = ko.observable()
  };
  metadataStore.registerEntityTypeCtor('Account', accountCtor);
  </code></pre>
  
  <p>How to omit this behaviour?</p>