Prevent SelfDrop context mutation across render boundaries (#2082)

lib/liquid/self_drop.rb

@@ -16,23 +16,25 @@ module Liquid
   #   then the local value takes precedence over the `self` object.
   # @liquid_access global
   class SelfDrop < Drop
-    def initialize(context)
+    def initialize(self_context)
       super()
-      @context = context
+      @self_context = self_context
     end
 
     def [](key)
-      @context.find_variable(key)
+      @self_context.find_variable(key)
     rescue UndefinedVariable
       nil
     end
 
     def key?(key)
-      @context.variable_defined?(key)
+      @self_context.variable_defined?(key)
     end
 
     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