title: mobx-state-tree: How to clone a model that includes references?
body: <p>I'm trying to clone a model that references another model, but I get: <code>Error: [mobx-state-tree] Failed to resolve reference 'H1qH2j20z' to type 'AnonymousModel' (from node: /usualCustomer)...</code> in the clone. The original resolves okay.</p>
  
  <p>Here are my models:</p>
  
  <pre><code>const Job = types.model({
      id: types.optional(types.identifier(types.string), shortid.generate()),
      jobNumber: types.optional(types.string, ''),
      description: '',
      usualCustomer: types.maybe(types.reference(Customer)),
    })
  
  const Customer = types.model({
      id: types.optional(types.identifier(types.string), shortid.generate()),
      name: types.optional(types.string, 'New customer'),
    })
  </code></pre>
  
  <p>This function shows the problem:</p>
  
  <pre><code>editJob = job =&gt; {
      console.log('Original', job)
      var newClone = clone(job)
      console.log('Clone', newClone)
  }
  </code></pre>