_parseHeaders(headers) {
            var parsed = {}, key, val, i;

            if(!headers) return parsed;

            forEach(headers.split('\n'), (line) => {
                i = line.indexOf(':');
                key = line.slice(0, i).trim().toLowerCase();
                val = line.slice(i + 1).trim();

                if(key) {
                    parsed[key] = parsed[key] ? parsed[key] + ', ' + val : val;
                }
            });

            return parsed;
        }
Parsed response headers
@param headers
@returns {Object}
@see https://github.com/angular/angular.js/blob/master/src/ng/http.js
@private