How to produce an error:
========================
It's quite easy to get 400, if you put a wrong mapping to your
index. That would be easy to reproduce in "normal" omelasticsearch usage
conditions, by only altering the ES configuration:

1. Make your index first. Let's call it "testindex":
$ curl -XPUT localhost:9200/testindex/

2. Put your mapping for a search type called "mytype", where you specify
that date property should be an integer:
$ curl -XPUT localhost:9200/testindex/mytype/_mapping -d '{"mytype":{"properties": {"timegenerated":{"type":"integer"}}}}'

3. Now try to insert something where date is not an integer:
$ curl -XPOST localhost:9200/testindex/mytype/ -d '{"timegenerated":"bla"}'
{"error":"MapperParsingException[Failed to parse [date]]; nested: NumberFormatException[For input string: \"bla\"]; ","status":400}

