Adopt 'undef context='

lib/liquid/self_drop.rb

@@ -31,10 +31,10 @@ module Liquid
       @self_context.variable_defined?(key)
     end
 
-    undef context
-
     def to_liquid
       self
     end
+
+    undef context=
   end
 end

lib/liquid/template.rb

@@ -151,8 +151,10 @@ module Liquid
 
         c
       when Liquid::Drop
-        drop         = args.shift
-        drop.context = Context.new([drop, assigns], instance_assigns, registers, @rethrow_errors, @resource_limits, {}, @environment)
+        drop = args.shift
+        c    = Context.new([drop, assigns], instance_assigns, registers, @rethrow_errors, @resource_limits, {}, @environment)
+        drop.context = c if drop.respond_to?(:context=)
+        c
       when Hash
         Context.new([args.shift, assigns], instance_assigns, registers, @rethrow_errors, @resource_limits, {}, @environment)
       when nil