This PR was merged into the 3.3-dev branch.

Discussion
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301 status code must drop request method to GET.

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| BC breaks?    | yes
| Tests pass?   | yes
| Fixed tickets | #20924
| License       | MIT

[RFC 7231 §6.4.2](https://tools.ietf.org/html/rfc7231#section-6.4.2) states that 301 HTTP Code should forward POST requests to the Location URI.

But, it also states that:

> For historical reasons, a user agent MAY change the request method from POST to GET for the subsequent request.

This is the behavior implemented in almost all user agents.
However the `BrowserKit` did forward the method to the subsequent request.

This PR make the `BrowserKit` change the request method from POST to GET when the response status code is 301.

Commits
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abda966d75 301 status code must drop request method to GET.