* Fix for stripping of 'Content-Disposition' and other headers from 'X-Accel-Redirect' redirect scripts.
* Added test case for header manipulation of redirect response.
- Server types no longer need to store their own contexts; they are
stored on the caddy.Instance, which means each context will be
properly GC'ed when the instance is stopped. Server types should use
type assertions to convert from caddy.Context to their concrete
context type when they need to use it.
- Pass the entire context into httpserver.GetConfig instead of only the
Key field.
- caddy.NewTestController now requires a server type string so it can
create a controller with the proper concrete context associated with
that server type.
Tests still need more attention so that we can test the proper creation
of startup functions, etc.
