Make require.stub() optionally accept a factory function

require.stub() can now accept a factory function instead of an object
so that stubbed modules are initialized lazily:

require.stub('zlib', function() {
    // initialized once, when zlib first required
    return {
        createGzip: function() { ... }
    };
});

http://code.google.com/p/phantomjs/issues/detail?id=1044

src/bootstrap.js

@@ -276,6 +276,9 @@ phantom.callback = function(callback) {
 
         // first see if there are any stubs for the request
         if (this.stubs.hasOwnProperty(request)) {
+            if (this.stubs[request].exports instanceof Function) {
+                this.stubs[request].exports = this.stubs[request].exports();
+            }
             return this.stubs[request].exports;
         }
 

test/require/require_spec.js

@@ -86,6 +86,23 @@ describe("require()", function() {
                 require('stubbed');
             }).should.Throw("Cannot find module 'stubbed'");
         });
+
+        describe("when invoked with a factory function", function() {
+            var count = 0;
+            require.stub('lazily_stubbed', function() {
+                ++count;
+                return 'lazily stubbed module';
+            });
+
+            it("initializes the module lazily", function() {
+                require('lazily_stubbed').should.equal('lazily stubbed module');
+            });
+
+            it("doesn't reinitialize the module each time it's required", function() {
+                require('lazily_stubbed');
+                count.should.equal(1);
+            });
+        });
     });
 
     describe("when the path is relative", function() {