fix bug in generate_known_helpers_hash

was previously undetected due to a bug in Handlebars:
handlebars-lang/handlebars.js#302

Author: turadg

lib/handlebars_assets/config.rb

@@ -70,6 +70,7 @@ def template_namespace
     def generate_known_helpers_hash
       known_helpers.inject({}) do |hash, helper|
         hash[helper] = true
+        hash
       end
     end