Like Jay Taylor said, HAProxy 1.5-dev has the redirect scheme
configuration directive, which accomplishes exactly what you need.
However, if you are unable to use 1.5, and if you're up for compiling HAProxy from source, I backported the redirect scheme
functionality so it works in 1.4. You can get the patch here: http://marc.info/?l=haproxy&m=138456233430692&w=2