Return proxied response body for all status codes

[mosiac1]

Reading a missing bucket/key will result in a 500 with the error "Insufficient content written 0 < x" (where X is a the expected content size) which seems to be a Jetty error.

The aws-proxy correctly logs that the response from MinIO has status 404 and the Content-Length header is set. With a breakpoint I confirmed that the response from MinIO has the same size as Content-Length, so I'm not sure why Jetty doesn't get any content.

Will continue investigating.

[mosiac1]

Found that StreamingResponseHandler did not stream the body of non-2XX responses but would send the original Content-Length header which would have a non-0 value, triggering the Jetty error that not enough content was written.

Updated StreamingResponseHandler to always include the body and added tests for all status codes that the S3 spec lists.

Created #154

[vagaerg]

LGTM