Default bundle.js to utf-8 encoding

I wanna run [caolan/forms validator tests](https://github.com/caolan/forms/blob/master/test/test-validators.js#L73) in testling, but some of the test data is unicode.  Boom.  

Adding an encoding to content-type header fixed up my problem.  It might make sense for the encoding to be configurable, but I'd wait for someone to make noise before adding the complexity.