Fallback to default_content if wildcard present in Accept header

If a browser makes a request like this:

    Accept: text/csv,*.*

And the application is implemented like this:

    set :default_content, :html

    get "/" do
      respond_to do |wants|
        format.html { erb :home }
      end
    end

The request would return a 404, even though the caller specifically said
it was ready to accept any mime type.

I found this when building an API that does not return HTML: it returns
JSON or CSV, but browsers request HTML, XHTML and */*, in this order. By
catering to what the caller asked for, the API is more easily explorable
in a browser.