fix custom demos (#2218)

Co-authored-by: jneen <jneen@jneen.net>

Author: jneen

lib/rouge/lexer.rb

@@ -124,14 +124,14 @@ def option(name, desc)
       def demo_file(arg=:absent)
         return @demo_file = Pathname.new(arg) unless arg == :absent
 
-        @demo_file = Pathname.new(File.join(__dir__, 'demos', tag))
+        @demo_file ||= Pathname.new(File.join(__dir__, 'demos', tag))
       end
 
       # Specify or get a small demo string for this lexer
       def demo(arg=:absent)
         return @demo = arg unless arg == :absent
 
-        @demo = File.read(demo_file, mode: 'rt:bom|utf-8')
+        @demo ||= File.read(demo_file, mode: 'rt:bom|utf-8')
       end
 
       # @return a list of all lexers.

spec/lexer_spec.rb

@@ -243,4 +243,12 @@ def initialize(*)
     assert { php.instance_variable_get(:@start_inline) == :guess }
     assert { inline_php.instance_variable_get(:@start_inline) == true }
   end
+
+  it 'supports custom demos' do
+    lexer = Class.new(Rouge::Lexer) do
+      demo 'my cool demo'
+    end
+
+    assert { lexer.demo == 'my cool demo' }
+  end
 end