Add have_http_link_header matcher and set header values as strings (#31010)

app/controllers/concerns/account_controller_concern.rb

@@ -20,7 +20,7 @@ module AccountControllerConcern
         webfinger_account_link,
         actor_url_link,
       ]
-    )
+    ).to_s
   end
 
   def webfinger_account_link

app/controllers/concerns/api/pagination.rb

@@ -19,7 +19,7 @@ module Api::Pagination
     links = []
     links << [next_path, [%w(rel next)]] if next_path
     links << [prev_path, [%w(rel prev)]] if prev_path
-    response.headers['Link'] = LinkHeader.new(links) unless links.empty?
+    response.headers['Link'] = LinkHeader.new(links).to_s unless links.empty?
   end
 
   def require_valid_pagination_options!