Fix fallback for browserify for optional modules.

Hello. In the commit 811dddc4090b4a6d69d470d356f35756eb03e340 there was a change that made request incompatible with browserify again. The error being thrown is `"Cannot read property 'require' of undefined"`. Not sure if this is the best way to fix it, but it sure fixes it :)

If I have the time one of these days (and you think it is a good idea, and want to keep supporting a browserified request) I could try to look into writing a small test-suite for the browser, so at least these things will get picked up by CI.

Also. I think I said it before, but thanks again for this awesome module! Please tell me if you want anything changed with the patch, and I'll fix it up.