Updated Memory.merge to handle null values

Previously, if the Memory store was merged with an object containing a null value, the following Error occurred:

TypeError: Object.keys called on non-object
    at Function.keys (native)
    at Memory.merge (/.../node_modules/nconf/lib/nconf/stores/memory.js:199:17)
    at Memory.merge (/.../node_modules/nconf/lib/nconf/stores/memory.js:200:17)
    at Array.every (native)
    at Memory.merge (/.../node_modules/nconf/lib/nconf/stores/memory.js:199:29)
    at common.merge (/.../node_modules/nconf/lib/nconf/common.js:99:13)
    at Array.forEach (native)
    at common.merge (/.../node_modules/nconf/lib/nconf/common.js:98:22)
    at Array.forEach (native)
    at Object.common.merge (/.../node_modules/nconf/lib/nconf/common.js:97:8)

This commit prevents that.

lib/nconf/stores/memory.js

@@ -157,7 +157,7 @@ Memory.prototype.merge = function (key, value) {
   // If the key is not an `Object` or is an `Array`,
   // then simply set it. Merging is for Objects.
   //
-  if (typeof value !== 'object' || Array.isArray(value)) {
+  if (typeof value !== 'object' || Array.isArray(value) || value === null) {
     return this.set(key, value);
   }