Provide expectations for controller specs

This commit adds expectations to AdminPublicBodyCategoriesController and
AdminPublicBodyHeadingsController edit actions
Redirect to #edit after updating

On a successful update AdminPublicBodyHeadingsController and
AdminPublicBodyCategoriesController #update should redirect to the #edit
action rather than simply rendering the ‘edit’ template.

Fixes https://github.com/mysociety/alaveteli/issues/1977
Redirect to #edit after updating

On an unsuccessful update AdminPublicBodyHeadingsController and
AdminPublicBodyCategoriesController #update should render the #edit
action.
Add expectation to #index spec

The spec that covers AdminPublicBodyCategoriesController#index did not
have an expectation
Add coverage to ensure ‘new’ templates are rendered
Add specs to cover unsuccessful creation
Use flash[:error] for error condition

Also use double quotes rather than string escaping
Merge branch '1977-update-categories-redirect-to-edit' into rails-3-develop