Fix exposed requires being persisted across bundles

As mentioned in the comments on issue #908 https://github.com/substack/node-browserify/issues/908#issuecomment-55956552, `expose`d files were being persisted across calls to browserify. I tracked this down to a leak where modules were being appended to the (module) global builtins object. Fixed with a test, though I'm not sure if you want a test for such a fine grained bug, so feel free to remove it, but it does demonstrate the failure & subsequent pass with this fix.