lib/common/index.js
function (options, must) {
    if (must) {
      for (var prop in must) {
        if (options[prop] === void 0 || options[prop] === null) {
          options[prop] = must[prop];
        }
      }
    }
    return options;
  }
function (options, must) {
    if (must) {
      for (var prop in must) {
        if (options[prop] === void 0 || options[prop] === null) {
          options[prop] = must[prop];
        }
      }
    }
    return options;
  }
Ensures that options at least contains propties and values from must
if they're not already defined and not null.
Differs from underscore by replacing
undefined *or null* falsies, and only one defaults source allowed.