closing fd's is sometimes helpful

We don't have connection pooling for the client anyway so at least don't
be so buggy and close fd's after every request.
Yep! I never intended for trunk to accumulate patches this long.  Conduit is necessary for Mirage HTTP(S) requests to work though, so lots of combinations to test (and a DNS resolver needs to be hooked in too, which is ongoing in mirage/ocaml-dns)