removed check for availability of json constants

The check for the availability of JSON_UNESCAPED_SLASHES is redundant
because we already require php >= 5.5 in composer.json and the constant
was introduced in PHP 5.4.

Also an check for the availability of ext-json was added to ensure
functions like json_encode() and the json constants are available.

Author: Michael Kühn

composer.json

@@ -28,6 +28,7 @@
     },
     "require": {
         "php": ">=5.5",
+        "ext-json": "*",
         "zendframework/zend-eventmanager": "~2.3",
         "zendframework/zend-http": "~2.3",
         "zendframework/zend-json": "~2.3",

src/ApiProblemResponse.php

@@ -23,7 +23,7 @@ class ApiProblemResponse extends Response
      *
      * @var int
      */
-    protected $jsonFlags = JSON_PARTIAL_OUTPUT_ON_ERROR;
+    protected $jsonFlags;
 
     /**
      * @param ApiProblem $apiProblem
@@ -34,9 +34,7 @@ public function __construct(ApiProblem $apiProblem)
         $this->setCustomStatusCode($apiProblem->status);
         $this->setReasonPhrase($apiProblem->title);
 
-        if (defined('JSON_UNESCAPED_SLASHES')) {
-            $this->jsonFlags |= constant('JSON_UNESCAPED_SLASHES');
-        }
+        $this->jsonFlags = JSON_UNESCAPED_SLASHES | JSON_PARTIAL_OUTPUT_ON_ERROR;
     }
 
     /**