guzzlehttp/psr7 1.6.1

Revert PSR-7 type assertions

on PHP Packagist

1.6.0 introduced a few type assertions to enforce types defined by PSR-7. Unfortunately that break the library for users not strictly following the PSR-7 standard. Since the users impacted by this change seems to be rather large, this hotfix reverts that change with the note that we will reapply it in 2.0.0, so fixing this is recommended regardless of which version you use.

Details are in #282 and #283