fix code to actually be compatible with psr/http-message 2

[dbu]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	fixes #153
Documentation	-
License	MIT

What's in this PR?

It seems the CI previously did not install with psr/http-message 2 with the return type declarations. Fix our code to have the types.

TODO

• [x] figure out why the CI still does not install with psr/http-message 2.0
• [x] Build for PSR-7 2.0 without slim
• [x] Adjust the code to be compatible with PSR-7 2.0

[stefangr]

Sadly it's not only the close method.

The slim/slim dev dependency requires psr/http-message ^1.