Refactor `Hobbit::Base#halt`. `Hobbit::Response` headers and body are not accessors anymore.

* Refactor `Hobbit::Base#halt`. It now sets the status, merges the headers and
writes the body (using `Hobbit::Response#write`) when given a fixnum, a hash or
a string.
* `Hobbit::Response` headers and body are not accessors anymore. This is
because when you set the body directly, the `Content-Length` is not calculated
(it's calculated on `#write`).