Adds zend-diactoros v2 support

Updated composer.json dev dependencies to run tests with zend-diactoros v2

Author: michalbundyra

composer.json

@@ -27,7 +27,7 @@
     "require-dev": {
         "phpunit/phpunit": "^7.0.2",
         "zendframework/zend-coding-standard": "~1.0.0",
-        "zendframework/zend-diactoros": "^1.7"
+        "zendframework/zend-diactoros": "^1.7 || ^2.1.1"
     },
     "autoload": {
         "psr-4": {