Copy the ivars from the sinatra route to the mustache

This makes your mustache behave more accordingly with what you'd expect
from a sinatra view: if you set an ivar on the route, you want it
available on your mustache.

lib/mustache/sinatra.rb

@@ -42,6 +42,10 @@ def render_mustache(template, data, options, locals, &block)
         instance = Mustache.new
       end
 
+      instance_variable_names.each do |name|
+        instance.instance_variable_set(name, instance_variable_get(name))
+      end
+
       locals.each do |local, value|
         instance[local] = value
       end