_assertHeader(header, res) {
    const field = header.name;
    const actual = res.header[field.toLowerCase()];
    const fieldExpected = header.value;

    if (typeof actual === 'undefined') return new Error('expected "' + field + '" header field');
    // This check handles header values that may be a String or single element Array
    if ((Array.isArray(actual) && actual.toString() === fieldExpected)
      || fieldExpected === actual) {
      return;
    }
    if (fieldExpected instanceof RegExp) {
      if (!fieldExpected.test(actual)) {
        return new Error('expected "' + field + '" matching '
          + fieldExpected + ', got "' + actual + '"');
      }
    } else {
      return new Error('expected "' + field + '" of "' + fieldExpected + '", got "' + actual + '"');
    }
  }
Perform assertions on a response header and return an Error upon failure.

@param {Object} header
@param {Response} res
@return {?Error}
@api private