Allow use of createForm() instance with <Form/>

[jamesdphillips]

Fixes unexpected error when using <Form /> component with an instance of the form api.

return (
  <Form
      form={form}
      render={...}
  />
)

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Closes #910

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Signed-off-by: James Phillips jamesdphillips@gmail.com

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[YegorMy]

That's a pretty important thing for me at the moment. Can we merge that one in if it looks good?