fix the missing parameters in the play processor

propositional_header and header_class were both missing from the top_of_page
declaration. This was stopping the template from version 0.7.1 to 0.7.2 from
compiling.

I've also renamed their contents to fit scala naming conventions and defaulted
them to `Html.empty` to make it easier for people to upgrade.

I'd like to rewrite the play processor to generate the top_of_page declaration
based on the other items but I don't really know enough ruby to know where to
start.

Also there should be tests here but I don't know enough ruby to know how to
test to ensure any yeilds that do return a value are listed in the top_of_page,
to prevent this happening again.
adding missing htmlLang to the top_of_page and including all defined in yeild in the content_for?
Merge pull request #76 from alphagov/fixPlayMissingParameters

Fix the missing parameters in the play processor