Do not compress for Cache-Control: no-transform by default

[dougwilson]

There has been a few questions raised about how a software project can opt-out of compression transparently, without requiring a user to configure this middleware to avoid the other project's output or have the other project be aware of compression. The current idea is that compression can, when determining to compress, check if there is a Cache-Control response header and if it has no-transform declared, skip compression.