unique "key" prop warning when running the tests

[faassen]

When I run the tests with yarn test I get this message on top of the test output:

Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of `ObjectFieldTemplate`. See https://fb.me/react-warning-keys for more information.
    in div (created by ObjectFieldTemplate)
    in ObjectFieldTemplate (created by ObjectField)
    in ObjectField (created by SchemaField)
    in div (created by DefaultTemplate)
    in DefaultTemplate (created by SchemaField)
    in SchemaField (created by Form)
    in form (created by Form)
    in Form

I traced this to ObjectFieldTemplate_test.js, line 33. It maps over properties like this:

            {properties.map(({ content, index }) => (
              <div key={index} className="property">
                {content}
              </div>
            ))}

But index doesn't appear to be a field of properties items. If I change the code to this the error goes away and the tests still pass:

            {properties.map(({ content }, index) => (
              <div key={index} className="property">
                {content}
              </div>
            ))}

If this looks all right I can make a pull request.