Ensure eval knows its whereabouts (#426)

Use #each_key, add a separating newline, and meaningfully name heredoc markers.

lib/sitemap_generator/templates.rb

@@ -9,17 +9,18 @@ module SitemapGenerator
   # Define an accessor method for each template file.
   class Templates
     FILES = {
-      :sitemap_sample =>  'sitemap.rb',
+      :sitemap_sample => 'sitemap.rb',
     }
 
     # Dynamically define accessors for each key defined in <tt>FILES</tt>
     attr_accessor(*FILES.keys)
-    FILES.keys.each do |name|
-      eval <<-END
+
+    FILES.each_key do |name|
+      eval(<<-ACCESSOR, binding, __FILE__ , __LINE__ + 1)
         define_method(:#{name}) do
           @#{name} ||= read_template(:#{name})
         end
-      END
+      ACCESSOR
     end
 
     def initialize(root = SitemapGenerator.root)