Fixed crashing when request body is not a JSON object

[edmocosta]

What does this PR do?

The JSON specification defines single values as valid JSON, it can be a string in double quotes, a number, true or false, or null. When the body is parsed, those values are transformed into their corresponding types. When those types aren't a Hash (aka JSON object), it breaks the LogStash::Event constructor, and Logstash crashes and exits.

After this PR, any request in which the body payload is a minimal JSON value, will be processed and an extra tag (_invalidjsonobject) will be added to the event:

LogStash::Event.new("message" => body, "tags" => "_invalidjsonobject")

Related issues

Closes #21

[andsel]

LGTM

---

Tested with

curl -L -H "Content-Type: application/json" -XGET "http://localhost:5010" -d '1'

pipeline:

input {
  github {
    codec => "json"
    port => 5010
  }
}

output {
  stdout{}