Fix tag in table_of_contents.html

RDoc::Markup::AttributeManager#convert_html that is called from
RDoc::Markup::AttributeManager#flow replaces HTML tags of received "str"
with "NULL" characters in HTML generation for each files, and the NULL
characters are removed in
RDoc::Markup::AttributeManager#split_into_flow. But the String objects
are reused for table_of_contents.html generation, so the tags lose at
that moment.

This commit fixes it with using duplicated String object.

Author: aycabta

lib/rdoc/markup/attribute_manager.rb

@@ -246,7 +246,7 @@ def add_special pattern, name
   # Processes +str+ converting attributes, HTML and specials
 
   def flow str
-    @str = str
+    @str = str.dup
 
     mask_protected_sequences
 

test/test_rdoc_markup_attribute_manager.rb

@@ -332,6 +332,14 @@ def test_protect
                   @am.flow("\\_cat_<i>dog</i>"))
   end
 
+  def test_lost_tag_for_the_second_time
+    str = "cat <tt>dog</tt>"
+    assert_equal(["cat ", @tt_on, "dog", @tt_off],
+                 @am.flow(str))
+    assert_equal(["cat ", @tt_on, "dog", @tt_off],
+                 @am.flow(str))
+  end
+
   def test_special
     @am.add_special(RDoc::CrossReference::CROSSREF_REGEXP, :CROSSREF)