POST /token HTTP/1.1
Content-Length: 91
user-agent: ericnorris/gcp-auth-contrib unit test
Host: oauth2.googleapis.com
Accept: application/json
Content-Type: application/x-www-form-urlencoded

grant_type=a-grant-type&target_audience=https%3A%2F%2Fexample.com&assertion-field=assertion