title: Does Apollo cache.readQuery return a copy?
body: <p>I was going through the <a href="https://www.apollographql.com/docs/react/essentials/mutations.html#update" rel="nofollow noreferrer">mutation doc</a>, and was wondering if <code>cache.readQuery</code> returns a copy?</p>
  
  <p>For instance:</p>
  
  <pre><code>const { todos } = cache.readQuery({ query: GET_TODOS });
  
  // Should I copy it first or is it already copied?
  // todos = Object.assign({}, todos)
  
  todos.items.concat([addTodo])
  cache.writeQuery({
            query: GET_TODOS,
            data: { todos: todos }
          });
  </code></pre>