ra_serf not storing client cert creds

I've noticed a feature that seems to be have been lost when we switched
from neon to serf: serf doesn't store the client cert creds. There are
two bits of data, the path to the pkcs12 file and the password for the
private key. The ra_serf library will prompt for these, if
config:auth:ssl-client-cert-file-prompt is set, but there is no code to
call svn_auth_save_credentials() and they do not get stored. The
ra_neon library did store them and all the relevant code is still
present in the providers.

I can't see any reason for dropping this feature so I think it should be
reinstated.

-- 
Philip