add defered closer calls for the http clients

[simskij]

add defered closer calls for the http client to avoid leaking file descriptors

[piksel]

This should at the very least apply best practice, and hopefully solve the reported FD leaks:

The default HTTP client's Transport may not reuse HTTP/1.x "keep-alive" TCP connections if the Body is not read to completion and closed.

It could be the case that the connections are being kept alive for re-use, but never actually reused because of this. But since the http.Client doesn't outlive the scope I don't think the connections would actually be reused, but that may still be the case due to some static logic in the underlying transports.
Edit: The DefaultTransport is static and reused for all http.Clients, so that is indeed the case.

Even if that were the case, this might still allow them to be closed when the client is pushed off the stack. The documentation only explicitly mentions that the Response.Body needs to be closed.