Fix link crawler not specifying accepted content-type (#12646)

The link crawler expects HTML documents, so set the `Accept`
header accordingly.

Fixes #12618

app/services/fetch_link_card_service.rb

@@ -45,7 +45,7 @@ class FetchLinkCardService < BaseService
   def html
     return @html if defined?(@html)
 
-    Request.new(:get, @url).perform do |res|
+    Request.new(:get, @url).add_headers('Accept' => 'text/html').perform do |res|
       if res.code == 200 && res.mime_type == 'text/html'
         @html = res.body_with_limit
         @html_charset = res.charset

app/services/fetch_oembed_service.rb

@@ -93,7 +93,7 @@ class FetchOEmbedService
   def html
     return @html if defined?(@html)
 
-    @html = @options[:html] || Request.new(:get, @url).perform do |res|
+    @html = @options[:html] || Request.new(:get, @url).add_headers('Accept' => 'text/html').perform do |res|
       res.code != 200 || res.mime_type != 'text/html' ? nil : res.body_with_limit
     end
   end