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]>

test/integration/context_test.rb

@@ -618,6 +618,20 @@ class ContextTest < Minitest::Test
     end
   end
 
+  def test_context_always_uses_static_registers
+    registers = {
+      my_register: :my_value,
+    }
+    c = Context.new({}, {}, registers)
+    assert_instance_of(StaticRegisters, c.registers)
+    assert_equal(:my_value, c.registers[:my_register])
+
+    r = StaticRegisters.new(registers)
+    c = Context.new({}, {}, r)
+    assert_instance_of(StaticRegisters, c.registers)
+    assert_equal(:my_value, c.registers[:my_register])
+  end
+
   private
 
   def assert_no_object_allocations

test/integration/standard_filter_test.rb

@@ -32,7 +32,7 @@ class TestDrop < Liquid::Drop
   attr_reader :value
 
   def registers
-    @context.registers
+    { @value => @context.registers[@value] }
   end
 end
 
@@ -440,7 +440,7 @@ class StandardFiltersTest < Minitest::Test
   end
 
   def test_map_calls_context=
-    model = TestModel.new(value: "test")
+    model = TestModel.new(value: :test)
 
     template = Template.parse('{{ foo | map: "registers" }}')
     template.registers[:test] = 1234