Make process.mixin copy over undefined values

This is not a bug in process.mixin, but I think it is undesirable
behavior. Right now process.mixin will not copy over keys with undefined
values. To me that is an unexpected filtering that should not happen
unless specifically called for.

src/node.js

@@ -137,7 +137,7 @@ process.mixin = function() {
 					, copy );
 
 				// Don't bring in undefined values
-				else if ( copy !== undefined )
+				else
 					target[ name ] = copy;
 
 			}

test/mjsunit/test-process-mixin.js

@@ -13,4 +13,10 @@ var fakeDomElement = {deep: {nodeType: 4}};
 target = {};
 process.mixin(true, target, fakeDomElement);
 
-assert.notStrictEqual(target.deep, fakeDomElement.deep);
+assert.notStrictEqual(target.deep, fakeDomElement.deep);
+
+var objectWithUndefinedValue = {foo: undefined};
+target = {};
+
+process.mixin(target, objectWithUndefinedValue);
+assert.ok(target.hasOwnProperty('foo'));