Unify body to buffer

[jimmywarting]

Forgot to make a PR.
Hade it ready two days ago

This remove some redundant code by normalizing the input body to buffer and eventually make it easier to make a stream getter out of it (res.body, req.body)

Report:

----------------|----------|----------|----------|----------|-------------------|
File            |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------------|----------|----------|----------|----------|-------------------|
All files       |    96.07 |    92.93 |      100 |    95.98 |                   |
 abort-error.js |      100 |      100 |      100 |      100 |                   |
 body.js        |    88.39 |    80.16 |      100 |       88 |... 16,320,321,326 |
 fetch-error.js |      100 |      100 |      100 |      100 |                   |
 headers.js     |      100 |      100 |      100 |      100 |                   |
 index.js       |      100 |      100 |      100 |      100 |                   |
 request.js     |    98.55 |    99.02 |      100 |    98.53 |               205 |
 response.js    |      100 |      100 |      100 |      100 |                   |
----------------|----------|----------|----------|----------|-------------------|

[bitinn]

LGTM, it's good that none of our tests depends on internal data structure.