Remove Content-Encoding header from HTTP 204 and 304 responses.

When using middleware which overrides content-related headers, such as gzip, Content-[Length|Type|MD5] is handled, but Content-Encoding was missed. As gzip content should either be returned in a chunked transfer encoding or with a content-length, several clients get confused because they set themselves up to perform decompression of a message which, per HTTP 204/304, has no message-body.