Ensure there is only one NAME section

Some documents may include a NAME section in the original text.  We'll
make sure to check before we add one of our own; if there's already
one, we can skip adding a new one.

lib/ronn/document.rb

@@ -460,7 +460,7 @@ def html_filter_inject_name_section
       markup =
         if title?
           "<h1>#{title}</h1>"
-        elsif name
+        elsif name && !@html.css('h2').map(&:text).include?('NAME')
           "<h2>NAME</h2>\n" \
           "<p class='man-name'>\n  <code>#{name}</code>" +
             (tagline ? " - <span class='man-whatis'>#{tagline}</span>\n" : "\n") +

test/existing_name_section.ronn

@@ -0,0 +1,9 @@
+# Test #
+
+## NAME ##
+
+test - the test manpage
+
+## DESCRIPTION ##
+
+Testing items.  w00t!

test/test_ronn_document.rb

@@ -200,4 +200,9 @@ def canonicalize(text)
     @doc = Ronn::Document.new('hello.1.ronn', styles: %w[test boom test]) { '' }
     assert_equal %w[man test boom], @doc.styles
   end
+
+  test 'NAME section is not duplicated' do
+    html = Ronn::Document.new(File.expand_path('existing_name_section.ronn', __dir__)).to_html
+    assert html.scan(/<h2[^>]*>NAME<\/h2>/).length == 1
+  end
 end