fall back to html if the format is something we cant handle

We've been seeing the following error with bad formats (like .php)

    …undle/ruby/2.1.0/gems/mobile-fu-1.3.1/lib/mobile-fu.rb: 129:in `set_mobile_format'
    …undle/ruby/2.1.0/gems/mobile-fu-1.3.1/lib/mobile-fu.rb: 104:in `set_request_format'
    …s/activesupport-3.2.19/lib/active_support/callbacks.rb: 451:in   `_run__3927665085735429995__process_action__462404612733862062__callbacks'
    ...

The root of the issue is that bad formats (I assume ones that mobile-fu is not looking for) don't respond to `html?`.  This fix falls back to html as the format if doesn't know how to ask `html?`.

I was unable to run the specs because `mocha/setup` was not available and I was not ready to debug the test suite.  I'm happy to add tests, if you can help get the framework working.

Do not use parentheses for method calls with no arguments.