fix issue #26

And related test.

When the client certificate chain contains a CA certificate, the
ngx_http_voms_module returns that one as the EEC.

This fix should resolve that issue.

Issue: https://baltig.infn.it/storm2/ngx_http_voms_module/issues/23

fix issue #19

fix issue #20

Add support for: user, user ca, server, server ca, server uri,
vo name, not before, not after, serial number, fqans.
Support for generic attributes requires a discussion on the
representation.