Refactored delegate implementing default and added tests

Author: damianopetrungaro

composer.json

@@ -3,7 +3,8 @@
   "require": {
     "php": ">=7.1",
     "psr/http-message": "^1.0",
-    "http-interop/http-middleware": "^0.4.1"
+    "http-interop/http-middleware": "^0.4.1",
+    "phpspec/prophecy": "^1.7"
   },
   "require-dev": {
     "phpunit/phpunit": "^6.2"