Fix behaviour of Context#stack when Context#push raises.

We only want to pop the scope at the end of the method if pushing it
succeeded, so the push needs to be outside the block with the ensure.

lib/liquid/context.rb

@@ -88,9 +88,12 @@ module Liquid
     #   context['var]  #=> nil
     def stack(new_scope={})
       push(new_scope)
-      yield
-    ensure
-      pop
+
+      begin
+        yield
+      ensure
+        pop
+      end
     end
 
     def clear_instance_assigns