Do not write LastHttpContent twice in HttpStaticFileServer example

Related: #3122

Motivation:

The HttpStaticFileServer example writes the LastHttpContent twice at the
end of the transfer.  HttpChunkedInput already produces a
LastHttpContent at the end of the stream, so there's no reason to write
another.

Modifications:

Do not write LastHttpContent in HttpStaticFileServerHandler when
HttpChunkedInput is used to transfer a file.

Result:

HttpStaticFileServer does not violates the protocol anymore.