Fix SelfDrop equality (#2091)

lib/liquid/context.rb

@@ -208,7 +208,7 @@ module Liquid
 
       # `self` resolves to a SelfDrop (enabling `self['var']` lookups),
       # but only when it hasn't been explicitly assigned as a local variable.
-      return SelfDrop.new(self) if key == Expression::SELF && !index
+      return @self_drop ||= SelfDrop.new(self) if key == Expression::SELF && !index
 
       variable = if index
         lookup_and_evaluate(@scopes[index], key, raise_on_not_found: raise_on_not_found)

lib/liquid/self_drop.rb

@@ -35,6 +35,20 @@ module Liquid
       self
     end
 
+    def ==(other)
+      other.is_a?(SelfDrop) && other.self_context.equal?(@self_context)
+    end
+
+    alias_method :eql?, :==
+
+    def hash
+      @self_context.object_id.hash
+    end
+
+    protected
+
+    attr_reader :self_context
+
     undef context=
   end
 end