REST Behat and Functional tests started to fail on 2.x/7.x once
kriswallsmith/buzz v0.17.0 got released introducing PSR-7 compliance. Due to SemVer, packages 0.x are not stable, so minor version releases can introduce BC breaks.

The quickest solution would be to force "~0.16.1" in composer, however that causes technical debt and could potentially block in the future developers wanting to use new PSR-7 standard.

It's a good chance to refactor the code for REST Functional tests and Behat BuzzDriver because:

• scope is relatively small, // ok, wasn't so small after all
• new changes to the library impose best practises and use of PSR7-related namespace and conventions.