add missing http body Close() in Client.GetLookup()

This was discovered with @grzesiek while trying to understand http connection churn as part of gitlab-com/gl-infra/production#1999 (closed).

We are not closing the request body after making API calls, which could lead to them not being reused by keepalive.

We are not entirely sure if this is the cause of the issue, as the stdlib http transport has some logic for detecting EOF.

It's probably a good idea to close the body in any case though.