The client may wait for the server to close the connection or for
timeout to occur in some cases. This commit changes it to always set
Content-Length field in non-100/204 responses regardless of whether
the body is empty. Although it doesn't cover all the patterns where
the response must not set it, users can remove it by calling
`set_content_length(None)`.

Signed-off-by: Takahiro Itazuri <itazur@amazon.com>