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.
[test] Added failing test to illustrate #65
[test] Better tests to show #65
[fix] Fix for #65
Merge pull request #67 from flatiron/gh-65

Fix for #65
[dist] Version bump. 0.6.6
Prefer this fix for #65 to 6045618
[dist] Version bump. 0.6.7
Added test to confirm merging an Object and null behaves as expected.
Merge pull request #63 from SchoonologyRRL/patch-1

Updated Memory.merge to handle null values