src/Http/ResponseParser.php
public static function json($response)
    {
        $body = self::toString($response);
        $data = json_decode($body, true);
        if (JSON_ERROR_NONE !== json_last_error()) {
            throw new RuntimeException('Unable to parse response body into JSON: ' . json_last_error());
        }
        return $data === null ? [] : $data;
    }
public static function json($response)
    {
        $body = self::toString($response);
        $data = json_decode($body, true);
        if (JSON_ERROR_NONE !== json_last_error()) {
            throw new RuntimeException('Unable to parse response body into JSON: ' . json_last_error());
        }
        return $data === null ? [] : $data;
    }
Parse the JSON response body and return an array

Copied from Response->json() in Guzzle3 (copyright @mtdowling)
@link https://github.com/guzzle/guzzle3/blob/v3.9.3/src/Guzzle/Http/Message/Response.php

@param  string|ResponseInterface $response
@throws RuntimeException if the response body is not in JSON format
@return array|string|int|bool|float