Do not use "Transfer-Encoding: chunked" header for proxied DELETE requests (again)

Created by: Turbo87

This issue is related to PR #373 where this exact bug was already fixed by a89e2f26, but then later apparently introduced again by 8663ac1c.

The actual problem is that DELETE requests with both Content-Length: 0 and Transfer-Encoding: chunked are illegal and are being rejected by some servers (in this case nodejs since v0.10.42). If the incoming request only has the Transfer-Encoding header the proxy will somehow automatically set the Content-Length header to 0 but won't remove the encoding header which leads to this issue.