By default, httpclient (and hence anything that uses rack-oauth2)
ignores the system-wide SSL certificate configuration in favor of its
own `cacert.pem`. This makes it impossible to use custom certificates
without patching that file. Until
https://github.com/nahi/httpclient/pull/386 is merged, we work around
this limitation by forcing the `HTTPClient` SSL store to use the default
system configuration.

Closes https://gitlab.com/charts/gitlab/issues/1436