websocket: add missing call to resp.Body.Close

Within hybiClientHandshake, after getting a response from
http.ReadResponse, the body is never closed.

When running a debugger with all of our test cases, this seems to not
matter usually since the body is typically a http.noBody, whose Close
just returns nil. However, this is not always the case. Therefore, this
CL adds the missing resp.Body.Close call.

Fixes golang/go#76952

Change-Id: I292d6ccc8eb101b806738aa7fc3e5446b623d861
Reviewed-on: https://go-review.googlesource.com/c/net/+/734240
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>