Charset fix and tests

Builds on top of #22!

DO NOT MERGE THIS UNTIL #22 IS MERGED :-)

Should fix #21. Part of https://trello.com/c/ObThuTMy/1755-proper-testsuite-for-api-blueprint-api-app and https://trello.com/c/CuPDy6Ve/1739-api-blueprint-api-error-handling-proper-http-codes.