topdown: Fix lost `tls_insecure_skip_verify` setting

If the `http.send` builtin is called with both `tls_insecure_skip_verify`
and another TLS configuration, the verificate setting will get lost as
the TLS configuration is overwritten. Ensure that we update the same
TLS configuration in all cases so that it is consistent.

Signed-off-by: James Peach <jpeach@vmware.com>