type/base: replace array_merge by + sign to merge a fallback array

array_merge resets the index, while in case of $this->colors and
$this->legend, we'd like to keep the index. An irq plugin sample:
array(1 => 1, 'ERR' => 'ERR') must not become
array(0 => 1, 'ERR' => 'ERR') after an array_merge.