(puppetlabs#302) Fix warnings generated by ERB.new

This has to special case Ruby <2.6.0 since those versions will interpret
the keyword argument as a hash passed as the second argument.

Without this change running this code in Ruby 2.6+ will generate
warnings about the parameters passed to `ERB.new`.

lib/puppet-strings/markdown/base.rb

@@ -182,8 +182,7 @@ def word_wrap(text, line_width: 120, break_sequence: "\n")
     # @return [String] full markdown rendering of a component
     def render(template)
       begin
-        file = File.join(File.dirname(__FILE__),"templates/#{template}")
-        ERB.new(File.read(file), nil, '-').result(binding)
+        PuppetStrings::Markdown.erb(template).result(binding)
       rescue StandardError => e
         fail "Processing #{@registry[:file]}:#{@registry[:line]} with #{file} => #{e}"
       end
@@ -196,4 +195,18 @@ def select_tags(name)
       tags.empty? ? nil : tags
     end
   end
+
+  # Helper function to load an ERB template
+  #
+  # @param [String] template The template name, including the .erb suffix.
+  # @return [ERB] Template
+  def self.erb(template)
+    template_path = File.join(File.dirname(__FILE__), 'templates', template)
+    if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.6.0')
+      ERB.new(File.read(template_path), trim_mode: '-')
+    else
+      # This works in Ruby 2.6+, but it outputs warnings.
+      ERB.new(File.read(template_path), nil, '-')
+    end
+  end
 end

lib/puppet-strings/markdown/table_of_contents.rb

@@ -17,8 +17,7 @@ def self.render
         group = toc
         priv = r.contains_private?
 
-        template = File.join(File.dirname(__FILE__),"templates/table_of_contents.erb")
-        final += ERB.new(File.read(template), nil, '-').result(binding)
+        final += PuppetStrings::Markdown.erb('table_of_contents.erb').result(binding)
       end
       final
     end