Add option not to parse response as JSON

[matus-tomlein]

It would be great if parsing responses as JSON could be made optional. I am accessing an API that accepts JSON in request body but responds only with the string "ok". This results in errors such as ERROR: ParseJSON: Unknown identifier 'ok'.

Could there be an option for it? Alternatively, the parsing could be dependent on the response content-type.

[TwitchBronBron]

@matus-tomlein We'd be happy to accept a pull request for this.

[iBicha]

@TwitchBronBron since the package is at 0.2.0, wouldn't it be better to introduce a breaking change, and make json into a method json()?