So you want to do this right?
WAN --> PROXY (cache) --> HTTP + HTTPS Backend
Usually, HTTPS content are secure, time sensitive, so instead of proxying, I have done the following myself in the past to reduce overhead.
WAN --> PROXY (cache) --> HTTP Backend
Plus one of the following:
1) WAN --> Have my router/firewall forward all HTTPS port content straight to --> HTTPS BACKEND bypassing the proxy altogether to reduce latency.
2) Or WAN --> Dedicated HTTPS hardware based compressor which interecept all HTTPS port action --> Proxy (cache) --> HTTP Backend.
This is assuming you do not need to HTTPS cached content. Do you need to HTTPS cached content?