Fix issue with wrong field name timeTracking (#116)

Wrong name leads to error when creating issues with the client, setting timeTracking property.

Error Message : {"errorMessages":[],"errors":{"timetTracking":"Field 'timetTracking' cannot be set. It is not on the appropriate screen, or unknown."}}

Author: ottschoNaud

src/Issue/IssueField.php

@@ -147,6 +147,11 @@ public function jsonSerialize()
             unset($vars['duedateString']);
         }
 
+        if (!empty($this->timeTracking)) {
+            $vars['timetracking'] = $this->timeTracking;
+            unset($vars['timeTracking']);
+        }
+
         return $vars;
     }