Content-Type improperly set to application/json

First, thanks for the package, it's very useful!

If I'm using [form-data](http://npm.im/form-data), the `Content-Type` is set to `application/json` even if I manually provide headers, seen here: https://github.com/amacneil/fetch-test-server/blob/master/index.js#L52-L55 

I think user-specified headers shouldn't be possible to override, but I'll leave that decision to you.