remove body on redirection if needed

[dbu]

Q	A
Bug fix?	yes
New feature?	yes
BC breaks?	no (except when relying on the behaviour)
Deprecations?	no
Related tickets	fixes #205
Documentation	php-http/documentation#298
License	MIT

What's in this PR?

Remove the body and content-type and content-length headers when following a redirection.

Why?

Sending a body to a different target than the intended one is usually not desired and can pose a security risk.

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created document stream_factory documentation#298

[GrahamCampbell]

This should only be done if there's a change in origin. The title mis-lead me. Having read over the diff, I see the purpose here is to remove the body on GET/HEAD redirect.

[dbu]

thanks. good point, i fixed the changelog to be clear about when we remove the body, and also explained it in the documentation