function toLowerCaseKeys_(obj) {
  if (obj === null || typeof obj !== 'object') {
    return obj;
  }
  // For each key in the source object, add a lower-case version to a new
  // object, and return it.
  return Object.keys(obj).reduce(function(result, k) {
    result[k.toLowerCase()] = obj[k];
    return result;
  }, {});
}
Gets a copy of an object with all the keys converted to lower-case strings.

@param {Object} obj The object to copy.
@return {Object} A shallow copy of the object with all lower-case keys.