Fix incorrect handling of relative favicon links

lib/frenzy/task/fetch_favicon.ex

@@ -35,7 +35,7 @@ defmodule Frenzy.Task.FetchFavicon do
 
         Logger.info("Trying default path: #{favicon_uri}")
 
-        case fetch_favicon_data(favicon_uri) do
+        case fetch_favicon_data(favicon_uri, site_url) do
           {:ok, favicon_data} ->
             changeset = Feed.changeset(feed, %{favicon: favicon_data})
             {:ok, _feed} = Repo.update(changeset)
@@ -49,7 +49,7 @@ defmodule Frenzy.Task.FetchFavicon do
   defp fetch_favicon_from_webpage(url) when is_binary(url) do
     case HTTP.get(url) do
       {:ok, %HTTPoison.Response{body: body}} ->
-        extract_favicon(body)
+        extract_favicon(url, body)
 
       {:error, _reason} = err ->
         err
@@ -58,7 +58,7 @@ defmodule Frenzy.Task.FetchFavicon do
 
   defp fetch_favicon_from_webpage(_), do: {:error, "URL must be a string"}
 
-  defp extract_favicon(body) do
+  defp extract_favicon(page_url, body) do
     html_tree = Floki.parse(body)
 
     case Floki.find(html_tree, "link[rel=icon]") do
@@ -95,13 +95,23 @@ defmodule Frenzy.Task.FetchFavicon do
             link
             |> Floki.attribute("href")
             |> List.first()
-            |> fetch_favicon_data()
+            |> fetch_favicon_data(page_url)
         end
     end
   end
 
-  defp fetch_favicon_data(url) when is_binary(url) do
+  defp fetch_favicon_data(favicon_url, site_url) when is_binary(favicon_url) do
-    case HTTP.get(url) do
+    absolute_url =
+      case URI.parse(favicon_url) do
+        %URI{host: nil, path: path} ->
+          # relative path
+          %URI{URI.parse(site_url) | path: path} |> URI.to_string()
+
+        _ ->
+          favicon_url
+      end
+
+    case HTTP.get(absolute_url) do
       {:ok, %HTTPoison.Response{body: body}} ->
         {:ok, "data:image/png;base64,#{Base.encode64(body)}"}
 
@@ -110,7 +120,7 @@ defmodule Frenzy.Task.FetchFavicon do
     end
   end
 
-  defp fetch_favicon_data(_), do: {:error, "No or invalid href for link"}
+  defp fetch_favicon_data(_, _), do: {:error, "No or invalid href for link"}
 
   # from https://github.com/elixir-plug/plug/blob/v1.8.3/lib/plug/request_id.ex#L60
   defp generate_task_id() do