Whenever we have an api that supports GET with a body, we always support the POST method too, as well as providing the body as a query_string parameter called `source`. Our REST spec should reflect this convention. FIxed them and introduced a hard check at parse time in our Java REST tests runner, which will cause the tests to fail if spec are not compliant.
Closes#9629
Files were generated with the `thor api:generate:spec --force --verbose --crawl` command,
and based on the 0.90 branch (elasticsearch/elasticsearch@5ec4064).