Content negotiation fix

Hi,

I've encountered two problems in `getResponseFormat()`'s handling of Accept headers with a wildcard.

The first issue is that requests with Accept `text/*` will cause Fatal error. `new XmlFormat` should be `new Format\XmlFormat`.

The second issue is that if my API only handles JSON (I have used `$restler->setSupportedFormats('JsonFormat');`, it will still respond with XML if a request has the Accept header `text/*`.

What do you think?