Skip prop-passing if route is a DOM element (fixes #226)

[developit]

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This fixes the following issue:

let div = render(
  <Router url="/?matches=hello">
    <div />
  </Router>
);

div.matches  // "hello" - should be DOM match() method

[developit]

Note to self: tests need to be updated to only use components as children, and to test that elements as children do not receive router props.