Don't try to run extension loaders unless they're actually functions

src/node.js

@@ -251,6 +251,7 @@ var module = (function () {
     this.filename = filename;
 
     var extension = path.extname(filename) || '.js';
+    if (!extensions[extension]) extension = '.js';
     extensions[extension](this, filename);
     this.loaded = true;
   };

test/fixtures/registerExt.hello.world

@@ -0,0 +1 @@
+exports.test = 'passed'

test/simple/test-module-loading.js

@@ -76,6 +76,8 @@ require.extensions['.test'] = function (module, filename) {
 };
 
 assert.equal(require('../fixtures/registerExt').test, "passed");
+// unknown extension, load as .js
+assert.equal(require('../fixtures/registerExt.hello.world').test, "passed");
 
 common.debug('load custom file types that return non-strings');
 require.extensions['.test'] = function (module, filename) {