Object metadata should be optional in resumable uploads

[coryan]

When starting a resumable upload the body with object metadata is optional. And at least the C++ client library takes advantage of this. The documentation is not explicit, but there are examples in the documentation that do not use a body:

https://cloud.google.com/storage/docs/performing-resumable-uploads#json-api

An equivalent request fails with fake-gcs-server:

$ curl --verbose -X POST -H 'Content-Length: 0' -H 'Content-Type: application/json'  'http://localhost:4443/storage/v1/b/fake-bucket/o?uploadType=resumable&name=fake-object'
*   Trying ::1:4443...
* Connected to localhost (::1) port 4443 (#0)
> POST /storage/v1/b/fake-bucket/o?uploadType=resumable&name=fake-object HTTP/1.1
> Host: localhost:4443
> User-Agent: curl/7.72.0
> Accept: */*
> Content-Length: 0
> Content-Type: application/json
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/plain; charset=utf-8
< X-Content-Type-Options: nosniff
< Date: Sat, 19 Sep 2020 13:21:25 GMT
< Content-Length: 4
< 
EOF