Implements ResourceInterface::availableMethods().
Overrides ResourceInterface::availableMethods
public function availableMethods() {
  $methods = $this
    ->requestMethods();
  $available = array();
  foreach ($methods as $method) {
    // Only expose methods where the HTTP request method exists on the plugin.
    if (method_exists($this, strtolower($method))) {
      $available[] = $method;
    }
  }
  return $available;
}