Make the citation controller's "show" action throw an

ActiveRecord::RecordNotFound exception when the given model is not found, just
as all the other controllers do.

This fixes issue PecanProject#668.

app/controllers/citations_controller.rb

@@ -123,7 +123,10 @@ def index
   # GET /citations/1
   # GET /citations/1.xml
   def show
-    @citation = Citation.where(:id => params[:id]).includes(params[:include]).first
+    # find_by! throws an ActiveRecord::RecordNotFound exception if no citation
+    # with the given id exists so that we don't attempt to display a nil
+    # citation.
+    @citation = Citation.includes(params[:include]).find_by!(:id => params[:id])
 
     respond_to do |format|
       format.html # show.html.erb