Remove 'Content-Length' header from a chunked HTTP response

Related: #3076

Motivation:

When a user writes a chunked HTTP response with a Content-Length header,
the HttpContentEncoder should remove the Content-Length header because
the length of the encoded content is supposed to be different from the
original one.

Actually, HttpContentEncoder currently never touches the Content-Length
header when the response is chunked.

Modifications:

- Remove the Content-Length header when an HTTP response being encoded
  is chunked
- Add a test case

Result:

HttpContentEncoder sanitizes the Content-Length header properly.