json parse fix

[vahpetr]

hotfix. need research how work origin fetch method json if server return text

[coveralls]

Coverage remained the same at 100.0% when pulling 82aa3e2 on vahpetr:master into 3c053ce on bitinn:master.

[bitinn]

Thx for the PR. But we have many discussion on this already, the answer remains: Fetch doesn't handle cases where server returns invalid response (if you got text when you are asking for json, it's invalid response).

EDIT: That includes empty string response on status 200 when you are asking for json. empty string is invalid json.