Rework JSON serialization (#94)

Instead of serializing JSON to a temporary slice of bytes before writing
to the response, instead write directly to the response.

Also, pretty-print JSON responses for better readability (and to match
httpbin).

Note: With this change, we rely much more on the underlying net/http
server's implicit handling of the Content-Length header. One notable
side effect of this is that responses to HEAD requests no longer include
a Content-Length.