fix(miniscore): _.defaults behavior for null (#41)

Adjust the _.defaults behavior to match that of underscore and lodash to
no longer treat `null` and `undefined` the same and only consider
`undefined` for override.

lib/hyperagent/miniscore.js

@@ -73,7 +73,7 @@ _.defaults = function (obj) {
   _.each(Array.prototype.slice.call(arguments, 1), function (source) {
     if (source) {
       for (var prop in source) {
-        if (obj[prop] === null || obj[prop] === undefined) {
+        if (obj[prop] === undefined) {
           obj[prop] = source[prop];
         }
       }