Fix Liquid::Template inheritance (#1227)

self.class.default_resource_limits would return `nil` in a subclass, since
the attribute isn't set on subclasses.

lib/liquid/template.rb

@@ -119,14 +119,13 @@ module Liquid
       # To enable profiling, pass in <tt>profile: true</tt> as an option.
       # See Liquid::Profiler for more information
       def parse(source, options = {})
-        template = Template.new
-        template.parse(source, options)
+        new.parse(source, options)
       end
     end
 
     def initialize
       @rethrow_errors  = false
-      @resource_limits = ResourceLimits.new(self.class.default_resource_limits)
+      @resource_limits = ResourceLimits.new(Template.default_resource_limits)
     end
 
     # Parse source code.

test/unit/template_unit_test.rb

@@ -77,4 +77,11 @@ class TemplateUnitTest < Minitest::Test
   ensure
     Template.tags.delete('fake')
   end
+
+  class TemplateSubclass < Liquid::Template
+  end
+
+  def test_template_inheritance
+    assert_equal("foo", TemplateSubclass.parse("foo").render)
+  end
 end