Allow HTTPlug 2 (and hence PSR-18)

[Jean85]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Documentation	not yet
License	MIT

What's in this PR?

This PR allows HTTPlug 2, and hence PSR-18. This is WIP to start a build and see if there is some compat issue to be resolved.

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

To Do

• Check the build
• Check if any doc PR is needed
• Check if any compat fix is needed

[Jean85]

Ok PR is complete! I'm not sure if a doc PR is needed though...

[dbu]

i don't see anything on http://docs.php-http.org/en/latest/discovery.html mentioning clients we return. and as httplug 1&2 are compatible from a consumer point of view, i think we need not do anything more really, lets merge.

[dbu]

we do not need a new release of discovery, right?

[Jean85]

Since this is a change in the composer.json to allow HTTPlug 2, yes @dbu, we need a new (minor?) release. That's the main reason that pushed me toward this PR 😄

[dbu]

the change is only in require-dev though, so its only relevant for the build of this repository, not for any consumers of discovery.

[Jean85]

Oh you're right, I didn't change anything outside of the test build! Good! 👍