HEAD requests seem to be broken

[richvdh]

rav@faith:~$ curl -I http://localhost:8008/_synapse/admin/v1/server_version
HTTP/1.1 400 Bad Request
Content-Type: application/json
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS
Content-Length: 73
Cache-Control: no-cache, no-store, must-revalidate
Date: Thu, 30 Jul 2020 15:06:09 GMT
Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept, Authorization
Server: Synapse/1.18.0rc2 (b=rav/fix_reject_no_extrems,b744eb3)

[clokep]

I went back to 1.6.1 (earliest I could easily get to run) and this seemed broken there.

I think it is straightforward though so I'll take a quick look.