Utilize ERB template in GistTag (#2785)

app/liquid_tags/gist_tag.rb

@@ -1,16 +1,18 @@
 class GistTag < LiquidTagBase
+  PARTIAL = "liquids/gist".freeze
+
   def initialize(tag_name, link, tokens)
     super
     @uri = build_uri(link)
   end
 
   def render(_context)
-    html = <<~HTML
-      <div class="ltag_gist-liquid-tag">
-          <script id="gist-ltag" src="#{@uri}"></script>
-      </div>
-    HTML
-    finalize_html(html)
+    ActionController::Base.new.render_to_string(
+      partial: PARTIAL,
+      locals: {
+        uri: @uri
+      },
+    )
   end
 
   def self.special_script

app/views/liquids/_gist.html.erb

@@ -0,0 +1,3 @@
+<div class="ltag_gist-liquid-tag">
+  <script id="gist-ltag" src="<%= uri %>"></script>
+</div>

spec/liquid_tags/gist_tag_spec.rb

@@ -40,14 +40,14 @@ RSpec.describe GistTag, type: :liquid_template do
     it "accepts proper gist url" do
       gist_links.each do |link|
         liquid = generate_new_liquid(link)
-        expect(liquid.render.delete(" ")).to eq(generate_script(link))
+        expect(liquid.render.tr("\n", " ").delete(" ")).to eq(generate_script(link))
       end
     end
 
     it "handles 'file' option" do
       liquid = generate_new_liquid(link_with_file_option)
       link, option = link_with_file_option.split(" ", 2)
-      expect(liquid.render.delete(" ")).to eq(generate_script(link, option))
+      expect(liquid.render.tr("\n", " ").delete(" ")).to eq(generate_script(link, option))
     end
 
     it "rejects invalid gist url" do