DOH POST frequently aborts with TCP connection closed

[honigpferd]

DOH POST requests frequently leads to TCP connection closed

How to reproduce

1. start DNS over HTTPS service from examples

2.  dig +https example.org +tries=1
   

observed result error log containing: TCP connection closed

reason Buffer.toString('utf8', ...) containing non-UTF8 characters leads to error

[gboudreau]

I had intermittent 502 errors when using DoH and your PR fixed it. Thanks!