expressにres.setとres.get追加

Added `res.set(field, val)` as an alternative to `res.header()` · expressjs/express@19a1a22 · GitHub
Added `res.get(field)` as an alternative to `res.header(field)` · expressjs/express@c8c6aa2 · GitHub

それぞれres.setHeaderとres.getHeaderのエイリアスっぽい。