Retain safe request method when following redirects

[webignition]

TLDR
A solution to #2046, ensures HEAD request method is retained when following 301 redirects.

Overview
RetryMiddleware::modifyRequest() modifies the method when following 301 redirects. In most cases, the method is changed to GET.

Safe methods (GET, HEAD, OPTIONS) can be retained safely when following redirects.

This change ensures that requests issued when following 301 redirects will retain the original request method if that method is safe.

[Chuvisco88]

Ran into the same issue (HEAD Request gets a 302 redirect and follows up with a GET Request).
This would fix it!
Why is it still open?! :(

[webignition]

@Chuvisco88 I'm not a maintainer and so can't merge. Maybe follow this up with @sagikazarmark?

[Nyholm]

This looks alright.

Thank you @Chuvisco88 for the review.

Thank you @webignition for the PR. Can you rebase this on master?

[webignition]

Yep, can rebase, no problem. I'll do so in about a week when I'm back from vacation.

[Nyholm]

Sorry for the long delay. Could you please rebase this PR?

[webignition]

@Nyholm Apologies also for not getting round to doing so years ago!

Rebased now, and as a bonus all the checks are passing 😄

[Nyholm]

Thank you

[gmponos]

Hi.. sorry for the delay.. I was 👍 on this

[albig]

@Nyholm, we would appreciate if this fix goes into a 6.5 bugfix release one day.

We use TYPO3 which has as composer requirement even in current master "guzzlehttp/guzzle: ^6.3.0"...