fix error in transform function when dealing with dropped entries (#287)

lib/nconf/common.js

@@ -164,7 +164,11 @@ common.transform = function(map, fn) {
   });
 
 
-  return pairs.reduce(function(accumulator, pair) {
+  return pairs
+    .filter(function(pair) {
+      return pair !== null;
+    })
+    .reduce(function(accumulator, pair) {
       accumulator[pair.key] = pair.value;
       return accumulator;
     }, {});

test/complete-test.js

@@ -214,6 +214,34 @@ vows.describe('nconf/multiple-stores').addBatch({
   teardown: function () {
     nconf.remove('env');
   }
+}).addBatch({
+  // Threw this in it's own batch to make sure it's run separately from the
+  // sync check
+  "When using env with a transform:fn that drops an entry": {
+    topic: function () {
+
+      function testTransform(obj) {
+        if (obj.key === 'FOO') {
+          return false;
+        }
+
+        return obj;
+      }
+
+      var that = this;
+      helpers.cp(complete, completeTest, function () {
+        nconf.env({ transform: testTransform });
+        that.callback();
+      });
+    }, "env vars": {
+      "port key/value properly transformed": function() {
+        assert.equal(typeof nconf.get('FOO'), 'undefined');
+      }
+    }
+  },
+  teardown: function () {
+    nconf.remove('env');
+  }
 }).addBatch({
   // Threw this in it's own batch to make sure it's run separately from the
   // sync check