Add PHP 8 support

Author: donatj

composer.json

@@ -10,15 +10,15 @@
 		}
 	],
 	"require": {
-		"php": "^7.1"
+		"php": "^7.1 | ^8.0"
 	},
 	"autoload": {
 		"psr-4": {
 			"Corpus\\Http\\": "src/"
 		}
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^7.5",
+		"phpunit/phpunit": "~7.5 | ~9.0",
 		"squizlabs/php_codesniffer": "^3.5",
 		"corpus/coding-standard": "^0.1.0",
 		"friendsofphp/php-cs-fixer": "^2.17"