Rails raises lame error when there is no block for custom mime types

Instead of giving up on custom mime types without blocks, Rails should
attempt to render the standard template. Currently you get this
useless exception, "Expected a block but none was given for custom
mime handler #{mime_type}"

I added a simple patch that tries to render the custom format's
template.

respond_to do |format|
  format.mobile # action.mobile.erb
  format.opml # action.opml.builder
end

Ticket #8174 (http://dev.rubyonrails.org/ticket/8174)