Fix unhandled "unshift is not a function" error

In certain cases, if there is only one entry for `memberOf`, the
attribute may be a string rather than an array. Attempting to call
`unshift` on the string object would cause an unhandled error. Added a
check to ensure that `memberOf` is an array before calling `unshift`,
and if it is a string, convert it into an array with the primary group
prepended.

lib/adauth.js

@@ -512,8 +512,10 @@ ADAuth.prototype._findGroups = function (obj, options, callback) {
 
           if (primaryGroup) {
             groups.unshift(primaryGroup);
-            if (obj.memberOf) {
+            if (Array.isArray(obj.memberOf)) {
               obj.memberOf.unshift(primaryGroup.dn);
+            } else if (obj.memberOf) {
+              obj.memberOf = [ primaryGroup.dn, obj.memberOf ];
             }
           }