bugfix: 'kid' not in given key list

if 'kid' value is not found in the given key map, should throw an exception.
Instead, it was outputting a php warning for using an undefined index, resulting in a null key.

src/JWT.php

@@ -98,6 +98,9 @@ public static function decode($jwt, $key, $allowed_algs = array())
         }
         if (is_array($key) || $key instanceof \ArrayAccess) {
             if (isset($header->kid)) {
+                if(!isset($key[$header->kid])) {
+                    throw new UnexpectedValueException('"kid" not found in key map, unable to lookup correct key');
+                }
                 $key = $key[$header->kid];
             } else {
                 throw new UnexpectedValueException('"kid" empty, unable to lookup correct key');