return Mime::NullType if format is unknown

If a request has an unknown format, the methods html?, xml?, json? ...etc
not raise an Exception.

This patch add a class Mime::NullType, that is returned when  request.format is unknown
and it responds false to the methods that ends with '?' and true to 'nil?'.

It refers to #7837, this issue is considered a improvement not a bug.

Conflicts:
	actionpack/CHANGELOG.md
	actionpack/lib/action_dispatch/http/mime_type.rb
Reverts rendering behavior when format is unknown

If a request has unknown format (eg. /foo.bar), the renderer
fallbacks to default format.

This patch reverts Rails 3.2 behavior after c2267db commit.

Fixes issue #9654.
Return nil for Mime::NullType#ref
Fix undefined method `ref' for nil:NilClass for bad accept headers

Conflicts:
	actionpack/CHANGELOG.md
	actionpack/lib/action_dispatch/http/mime_type.rb