Updated Memory.merge to handle null values
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.
ChristianMurphy/nconf,Dependencies/nconf,philip1986/nconf,NickHeiner/nconf,indexzero/nconf,imjerrybao/nconf,remy/nconf,bryce-gibson/nconf,HansHammel/nconf,olalonde/nconf