Match when a header is missing?

[Shawn Heisey]

I have a need to cause haproxy to match an ACL when a header 
(User-Agent) is missing.  Can that be written with the configuration 
language in its current state?  I'm running 1.4.18 here.

In another installation, I am running 1.5.dev7, modified with a BUFSIZE 
of 65536 because we have some really long URLs sent to Solr.  I'm having 
a problem where my checks are occasionally failing, but I don't see any 
apparent problem on the Solr servers.  I looked into upgrading recently 
to a newer dev snapshot, but I see that the download section says "may 
be broken" for 1.5dev12.  How is it broken?  Would I be OK running 
1.5dev12 as a front end for Solr servers?

Thanks,
Shawn