X-Content-Encoding-Over-Network in Response Header but not Content-Encoding
Your configuration is just good. Sometimes, the problem is caused by Antivirus just like mentioned in comment. Try disabling the antivirus or instead search for HTTP Scanning option in your antivirus program.
NOD Internet Security does this. You can disable this option by following,
1) Open NOD Internet Security
2) Click Setup then Advance Setup
3) Search for HTTP
4) Disable HTTP Scanner