fix isset to avoind null vs. missing value

src/Request/Request.php

@@ -96,7 +96,7 @@ public function getOptionalField($name, $default = null)
      */
     public function hasField($name)
     {
-        return isset($this->getDecodedJsonFromBody()->$name);
+        return array_key_exists($name, (array)$this->getDecodedJsonFromBody());
     }
 
     /**

tests/Request/RequestTest.php

@@ -9,14 +9,31 @@
 use Mockery;
 use Mockery\MockInterface;
 use PHPUnit_Framework_TestCase;
+use Psr\Http\Message\StreamInterface;
+use Slim\Http\Body;
+use Slim\Http\Headers;
 use Slim\Http\Request as SlimRequest;
+use Slim\Http\Uri;
 
 final class RequestTest extends PHPUnit_Framework_TestCase
 {
 
     const PARAM_NAME = 'dateFrom';
     const DATE_TIME_STRING = '2017-06-10T01:00:00+01:00';
 
+    public function testShouldDistinguishBetweenNullAndEmptyOption()
+    {
+        $body = new Body(fopen('php://temp', 'rb+'));
+        $body->write('{"thisIsNull": null, "thisIsGandalf": "gandalf"}');
+        $body->rewind();
+
+        $request = $this->createRequest($body);
+
+        $this->assertTrue($request->hasField('thisIsNull'));
+        $this->assertFalse($request->hasField('nonExistingField'));
+        $this->assertTrue($request->hasField('thisIsGandalf'));
+    }
+
     public function testGettingDateTimeQueryParam()
     {
         $arguments = [self::PARAM_NAME => self::DATE_TIME_STRING];
@@ -77,4 +94,16 @@ private function createMockSlimRequest(array $arguments)
         return $mock;
     }
 
+    /**
+     * @param StreamInterface $body
+     * @return Request
+     */
+    private function createRequest(StreamInterface $body)
+    {
+        $url = new Uri('https', 'example.com');
+        $slimRequest = new SlimRequest('POST', $url, new Headers(), [], [], $body);
+
+        return new Request($slimRequest);
+    }
+
 }