Version changing between POSTs results in loss of POST data

[0b10011]

Reproducing results

1. Load a multi-step form
2. Submit to go on to the second step
3. Change version sent in response header
4. Submit to go on to third step

Expected results

• POST data from first/second step should be retained

Actual results

• POST data from first/second step is lost

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A possible solution would be to send the version in a request header, require the server to check it before processing POST data (perhaps with a flag via response header to avoid repeating actions), and then resubmit the POST data the old-fashioned way. (See #357.)