Silence potentially upcoming circular dependency warning (#973)

Node.js is currently considering printing a warning when a non-existent
property of `module.exports` is accessed while in a circular `require()`
dependency, in order to make it easier to catch issues with circular
dependencies.

In order to avoid printing these warnings for shelljs, checking for the
property’s existence rather than its truthiness suffices.

Refs: nodejs/node#29935

Author: dabbertorre

src/common.js

@@ -478,7 +478,7 @@ function _register(name, implementation, wrapOptions) {
   // If an option isn't specified, use the default
   wrapOptions = Object.assign({}, DEFAULT_WRAP_OPTIONS, wrapOptions);
 
-  if (shell[name]) {
+  if (shell.hasOwnProperty(name)) {
     throw new Error('Command `' + name + '` already exists');
   }