Checking for paylods as JSON objects in completion suggester

If the payload is not a JSON object on indexation, an exception will be thrown.
Completion Suggester: Allow payload to be a value
closes #3550
Plugin Manager can not download _site plugins from github
Sounds like github changes a bit download url for master zip file.

From `https://github.com/username/reponame/zipball/master` to `https://codeload.github.com/username/reponame/zip/master`.

We need to update plugin manager to reflect that change.

In the meantime, we invite users having this issue to use:

```sh
bin/plugin -install reponame -url https://codeload.github.com/username/reponame/zip/master
```

For example:

```sh
bin/plugin -install paramedic -url https://codeload.github.com/karmi/elasticsearch-paramedic/zip/master
```

Closes #3551