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
dtto with `content-type` header