- fix proxy mode when 'redirect_to_exact=True' is set config, don't redirect in proxy mode
- more general prefer support, moved to content_rewriter to support preference<->mod mappings
- add 'banner-only' preference mapped to bn_ modifier
- proxy mode: allow 'raw' and 'banner-only' preferences
- proxy mode: 'Prefer: rewritten' forced to 'banner-only', served with 'Preference-Applied: banner-only'
- tests: test proxy with prefer header, 'redirect_to_exact=True', add 'banner-only' to Prefer header tests in rewriting mode