Ensure that partial caches are shared with subcontexts

Make Context use StaticRegisters by default. This makes it easier to
ensure that all subcontexts share the same static registers.

Co-authored-by: Dylan Thacker-Smith <[email protected]>

lib/liquid/context.rb

@@ -28,7 +28,7 @@ module Liquid
 
       @static_environments = [static_environments].flat_map(&:freeze).freeze
       @scopes              = [(outer_scope || {})]
-      @registers           = registers
+      @registers           = registers.is_a?(StaticRegisters) ? registers : StaticRegisters.new(registers)
       @errors              = []
       @partial             = false
       @strict_variables    = false
@@ -39,6 +39,10 @@ module Liquid
       @global_filter       = nil
       @disabled_tags       = {}
 
+      @registers.static[:cached_partials] ||= {}
+      @registers.static[:file_system] ||= Liquid::Template.file_system
+      @registers.static[:template_factory] ||= Liquid::TemplateFactory.new
+
       self.exception_renderer = Template.default_exception_renderer
       if rethrow_errors
         self.exception_renderer = Liquid::RAISE_EXCEPTION_LAMBDA