[API] Changed, that the `:ignore` parameter to APIs can be a scalar value or an Array

Thus, `subject.indices.delete index: 'foo', ignore: 404` and
`subject.indices.delete index: 'foo', ignore: [404]` are equivalent.

Related: elasticsearch/elasticsearch@8c7bbbcc8f52bec16a9afe05e5de2984916975df
[API] Added the "Pending Tasks" API

Related: elasticsearch/elasticsearch#3368

See: http://www.elasticsearch.org/guide/en/elasticsearch/reference/master/cluster-pending.html