Extra test for converting elements within arrays

Previously: "mutate { convert => [ \"[a][0]\", \"float\" ]  } }" would
throw an exception because the '0' element accessor was being treated
as a String by Logstash.

Now: A fix in Logstash should fix this, and this test is here to verify
that.


related issue: https://github.com/elasticsearch/logstash/issues/1401