Solr responded with an error (HTTP 400): [Reason: Error parsing JSON field value. Unexpected OBJECT_START at [##], field=??]

[FuhuXia]

I have

• Tested with the latest release
• Tested with the current master branch
• Searched for similar existing issues

Expected behaviour

No solr error when adding doc.

Actual behaviour

Error when doc contains multiple levels of nested objects.

Solr responded with an error (HTTP 400): [Reason: Error parsing JSON field value.
Unexpected OBJECT_START at [79], field=property_level1]

Steps to reproduce the behaviour

Add doc:

{
	"id": "doc_1",
	"title": "title_1",
	"property_level1": {
		"property_level2": {
			"property_level3": 1
		}
	}
}

Configuration

• Operating system version: Ubuntu 18.04
• Search engine version: 5.5.5
• Python version: 2.7.16
• pysolr version: 3.9.0

[FuhuXia]

Only happens on pysolr version 3.9.0. It is fine on 3.6.0, 3.7.0, 3.8.0.