Fix Exception thrown at ObjectSerializer::toString() (#1)

Allow Guzzle 7, prevent exception "Trying to access array offset on value of type int".

Author: BBrunekreeft

composer.json

@@ -29,7 +29,7 @@
         "ext-curl": "*",
         "ext-json": "*",
         "ext-mbstring": "*",
-        "guzzlehttp/guzzle": "^6.3"
+        "guzzlehttp/guzzle": "^6.3|7.*"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "2.10.*",
@@ -44,4 +44,4 @@
         "classmap": [ "src/", "tests/" ],        
         "psr-4": { "GroupDocs\\Conversion\\" : "tests/" }
     }
-}
+}

src/GroupDocs/Conversion/ObjectSerializer.php

@@ -169,12 +169,17 @@ public static function toFormValue($value)
      */
     public static function toString($value)
     {
-        if (date(\DATE_ATOM, preg_match("/^[1-9][0-9]*$/", $value)[0]) instanceof \DateTime) { // datetime in ISO8601 format
-            $datetime = preg_match("/^[1-9][0-9]*$/", $value)[0];
-            return date(\DATE_ATOM, $datetime);
-        } else {
+        preg_match("/^[1-9][0-9]*$/", $value, $matches);
+
+        if (empty($matches)) {
             return $value;
         }
+        
+        if (date(\DATE_ATOM, $matches[0]) instanceof \DateTime) { // datetime in ISO8601 format
+            return date(\DATE_ATOM, $matches[0]);
+        }
+
+        return $value;
     }
 
     /*