Fix `formData` Upload

We're trying to upload our Transloadit library to the latest over at Sprint.ly and we kept getting the following error:

![screen shot 2014-02-14 at 6 12 41 pm](https://f.cloud.github.com/assets/126919/2176921/15862938-95e7-11e3-91ce-0e834c2cce53.png)

And I checked line 290 where it checks for a `formData` option and posts via `XMLHttpRequest` and it was referencing a var, `b`, that was not set anywhere in-scope. I've pulled out the URL used for the `<form>` post method, and put it into a variable for both methods to reference and it seems to work now.

Please take a look at my changes and let me know if there's anything I'm missing/misunderstanding. And I don't see any unit tests in this repo, but I'm happy to write one if you've got an existing suite somewhere that I can add on to.

Thanks!