perf: set default html and json content types directly

[Abdel-Monaam-Aouini]

res.send(string) calls res.type('html') and res.json() sets application/json when no Content-Type is set. Both go through a mime lookup (mime.contentType) just to get back text/html; charset=utf-8 and application/json; charset=utf-8.

This sets those two values directly, so the header is the same but the lookup is skipped. A micro benchmark shows the lookup costs ~200ns for html and ~100ns for application/json. I did not see a difference in a full request benchmark, it is within noise, so this is a small saving per response.

Added tests that check the exact header value, that there is a single Content-Type header, HEAD requests, and that a type set earlier with res.type() is kept.