Remove lazy load stacks

Remove lazy load stacks and instead only create a new scope when a tag is known to need one

lib/liquid/context.rb

@@ -25,8 +25,6 @@ module Liquid
       @resource_limits  = resource_limits || ResourceLimits.new(Template.default_resource_limits)
       squash_instance_assigns_with_environments
 
-      @this_stack_used = false
-
       self.exception_renderer = Template.default_exception_renderer
       if rethrow_errors
         self.exception_renderer = ->(e) { raise }
@@ -111,19 +109,26 @@ module Liquid
     #   end
     #
     #   context['var]  #=> nil
-    def stack(new_scope = nil)
-      old_stack_used = @this_stack_used
-      if new_scope
-        push(new_scope)
-        @this_stack_used = true
-      else
-        @this_stack_used = false
-      end
-
+    #
+    # false or {} can be used to control if a new scope is needed
+    #
+    # Example:
+    #   new_scope = false
+    #   context.stack(new_scope) do
+    #      # no scope created
+    #   end
+    #
+    # Example:
+    #   new_scope = {}
+    #   context.stack(new_scope) do
+    #      # scope created
+    #   end
+    #
+    def stack(new_scope = {})
+      push(new_scope) unless new_scope == false
       yield
     ensure
-      pop if @this_stack_used
-      @this_stack_used = old_stack_used
+      pop unless new_scope == false
     end
 
     def clear_instance_assigns
@@ -132,10 +137,6 @@ module Liquid
 
     # Only allow String, Numeric, Hash, Array, Proc, Boolean or <tt>Liquid::Drop</tt>
     def []=(key, value)
-      unless @this_stack_used
-        @this_stack_used = true
-        push({})
-      end
       @scopes[0][key] = value
     end