Transports should allow setting DisableCompression

[julz]

/kind bug

Expected Behavior

The transports returned by NewAutoTransport should support disabling go's helpful functionality where it silently adds an Accept-Encoding: gzip header and uncompresses the result for you. While this feature is often useful, it's not what we want for Activator and QP's ReverseProxy transports since it (a) injects an extra header not in the client side in to the request the user container receives, and (b) causes the QP/Activator to end up uncompressing things uselessly and wastefully if the user container happens to co-operate with that request

Actual Behavior

The transports do not set DisableCompression=true, so Go helpfully injects an Accept-Encoding: gzip header. This ends up causing the user container to compress the response (if it supports gzip encoding), which the QP immediately then immediately uncompresses :).

Steps to Reproduce the Problem

1. ko apply ksvc.yaml from https://github.com/julz/gziphello
2. curl the container, don't pass any extra headers
3. see in logs that gzip header was nevertheless added
4. notice (by logging resp.Uncompressed) that queue proxy uncompressed the response and the container compressed it entirely pointlessly