add missing response fields

[kennedymwavu]

add these missing response fields:

• [x] set_status
• [x] md
• [x] png
• [x] jpeg
• [x] image
• [x] ggplot2
• [x] header_content_*
• [x] set_headers
• [x] pre_render_hook
• [x] post_render_hook
• [x] cookie
• [x] clear_cookie