Fix behaviors when headers are not present

Hi! I was using your module and I noticed we kept responding with 406 to a certain client. It turns out the client had a privacy proxy running and was not sending any `Accept` header with the request. I checked in RFC 2616 and apparently they have information on what should be done if the header is missing. I updated the different modules with the RFC 2616 recommendations and added tests for the missing header cases.

Since `new Negotiator(req)` will pass `req.headers[name]` down to the modules, a missing header would pass down `undefined`, so I added the support for `undefined` to all the modules (it makes sense anyway, because the acceptable media type is not defined :))

RFC 2616 14.1

> If no Accept header field is present, then it is assumed that the client accepts all media types.

RFC 2616 14.2

> If no Accept-Charset header is present, the default is that any character set is acceptable.

RFC 2616 14.3

> If no Accept-Encoding field is present in a request, the server MAY assume that the client will accept any content coding. In this case, if "identity" is one of the available content-codings, then the server SHOULD use the "identity" content-coding, unless it has additional information that a different content-coding is meaningful to the client. 

RFC 2616 14.4

> If no Accept-Language header is present in the request, the server SHOULD assume that all languages are equally acceptable.

Right now using this module as `new Negotiator(req)` results in a violation of section 14.1 and 14.2 of RFC 2616. Sections 14.3 and 14.4 are nice to have (and 14.3 already occurs, I just added a test for it).