Don't do array lookups on strings. (#188)

`hasOwnProperty(number)` can return true for strings.

This is unlikely to be the desired usage, and can mean that odd
responses are returned by nconf.

Disable trying to check `hasOwnProperty` of strings.

lib/nconf/stores/memory.js

@@ -44,7 +44,7 @@ Memory.prototype.get = function (key) {
   //
   while (path.length > 0) {
     key = path.shift();
-    if (target && target.hasOwnProperty(key)) {
+    if (target && typeof target !== 'string' && target.hasOwnProperty(key)) {
       target = target[key];
       continue;
     }

test/nconf-test.js

@@ -67,6 +67,9 @@ vows.describe('nconf').addBatch({
         "without a callback": {
           "should respond with the correct value": function () {
             assert.equal(nconf.get('foo:bar:bazz'), 'buzz');
+          },
+          "should not step inside strings": function () {
+            assert.equal(nconf.get('foo:bar:bazz:0'), undefined);
           }
         },
         "with a callback": {