Revert "Disable input buffering when 'Connection: Upgrade' header is set."

This reverts commit df8d020afd8c51998b96f3aac79764f09d321f69.

After some thinking this is not a good idea. An attacker can abuse this
feature to disable input buffering, resulting in a DoS. It's probably
best to require explicit disabling of input buffering in the config.