Allow installation of psr/http-message v2 (#28)

Closes #27

Author: gsteel

composer.json

@@ -11,7 +11,7 @@
     "require": {
         "php": "^8.0",
         "helmich/phpunit-json-assert": "^3.4",
-        "psr/http-message": "^1.0"
+        "psr/http-message": "^1.1 || ^2.0"
     },
     "require-dev": {
         "guzzlehttp/psr7": "^2.4",