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.

Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.