[4.0.0] UnicodeDecodeError: 'utf-8' codec can't decode byte 0x96 in position 91: invalid start byte

Django 1.11.16
APM agent 4.0.0
APM server 6.5

Exception is raised at https://github.com/elastic/apm-agent-python/blob/f600230a1643a9442605fce959946790c85c4c61/elasticapm/transport/http.py#L62

For my case `body` is the bytes string below:

    b'data read error: invalid character \'\\x00\' looking for beginning of value [\x00\x00P\x02\x00\x00\.....