WEBrick::HTTPProxyServer implementes HTTP proxy using
WEBrick and Net::HTTP.
WEBrick accepts HTTP/1.0 clients and
Net::HTTP uses always HTTP/1.1.
However HTTP/1.1 supports chunked transfer coding HTTP/1.0 doesn't.
Chunked transfer coding doesn't require that
content-length before the content is sent.
But non-chunked transfer coding require content-length before
the content is sent.
So, when HTTP/1.0 clients connects WEBrick::HTTPProxyServer and
origin server returns chunked response,
WEBrick::HTTPProxyServer needs to store whole content to
know the length of it.
This patch do it using tempfile.