Tests for composer

Built on top of #28.

PLEASE, DO NOT MERGE UNTIL #28 IS MERGED.

- https://trello.com/c/ObThuTMy/1755-proper-testsuite-for-api-blueprint-api-app
- https://trello.com/c/CuPDy6Ve/1739-api-blueprint-api-error-handling-proper-http-codes
please use this approach (e.g.)
```
contentTypeHeader = res.request.hasHeader('content-type', res.headers)
  if contentTypeHeader
    console.error 'content-type', res.headers[contentTypeHeader]
```