Missing fields in `MultiTermsAggregation`

`MultiTermsAggregation` is missing a number of fields:
* `collect_mode`,
* `order`,
* `min_doc_count`,
* `shard_min_doc_count`,
* `shard_size`,
* `show_term_doc_count_error`,
* `size`

Closes #310 reported by @andreluiz1987 by rewording it as an API-specification related issue.