Correct Cache setWorkarounds to take account of multiple script or st…

…yle declarations being used that mean that we have an array of scripts or stylesheets to be cached and not just a string

Cache setWorkarounds returns an array not a string

libraries/src/Cache/Cache.php

@@ -595,7 +595,7 @@ public static function getWorkarounds($data, $options = array())
 	 * @param   string  $data     Cached data
 	 * @param   array   $options  Array of options
 	 *
-	 * @return  string  Data to be cached
+	 * @return  array  Data to be cached
 	 *
 	 * @since   1.7.0
 	 */
@@ -688,9 +688,13 @@ public static function setWorkarounds($data, $options = array())
 								{
 									$oldScriptStr = $options['headerbefore'][$now][strtolower($type)];
 
-									if ($oldScriptStr != $currentScriptStr)
+									// Save only the appended declaration.
+									if (\is_array($oldScriptStr) && \is_array($currentScriptStr))
+									{
+										$newvalue[strtolower($type)] = array_diff_key($currentScriptStr, $oldScriptStr);
+									}
+									else
 									{
-										// Save only the appended declaration.
 										$newvalue[strtolower($type)] = StringHelper::substr($currentScriptStr, StringHelper::strlen($oldScriptStr));
 									}
 								}