fix: make sure we detect content type and set properly or allow it to…

… pass through if set

lib/request_cache/plug.ex

@@ -16,6 +16,8 @@ defmodule RequestCache.Plug do
   # This is compile time so we can check quicker
   @graphql_paths RequestCache.Config.graphql_paths()
   @request_cache_header "rc-cache-status"
+  @json_regex ~r/^(\[|\{)(.*|\n)*(\]|\})$/
+  @html_regex ~r/<!DOCTYPE\s+html>/i
 
   @impl Plug
   def init(opts), do: opts
@@ -95,10 +97,23 @@ defmodule RequestCache.Plug do
     conn
     |> Plug.Conn.halt()
     |> Plug.Conn.put_resp_header(@request_cache_header, "HIT")
-    |> Plug.Conn.put_resp_content_type("application/json")
+    |> maybe_put_content_type(result)
     |> Plug.Conn.send_resp(200, result)
   end
 
+  defp maybe_put_content_type(conn, result) do
+    case Plug.Conn.get_resp_header(conn, "content-type") do
+      [_ | _] -> conn
+      [] ->
+        cond do
+          result =~ @json_regex -> Plug.Conn.put_resp_content_type(conn, "application/json")
+          result =~ @html_regex -> Plug.Conn.put_resp_content_type(conn, "text/html")
+
+          true -> conn
+        end
+    end
+  end
+
   defp rest_cache_key(%Plug.Conn{request_path: path, query_string: query_string}) do
     Util.create_key(path, query_string)
   end

test/request_cache_plug_test.exs

@@ -162,6 +162,36 @@ defmodule RequestCachePlugTest do
            ]
   end
 
+  test "allows for for custom content-type header and returns it when served from the cache", %{
+    caller_pid: pid
+  } do
+    route = "/my_route/:param"
+    assert %Plug.Conn{resp_headers: uncached_headers} =
+             :get
+             |> conn(route)
+             |> RequestCache.Support.Utils.ensure_default_opts()
+             |> put_private(:call_pid, pid)
+             |> Router.call([])
+
+    assert uncached_headers === [
+       {"cache-control", "max-age=0, private, must-revalidate"}
+    ]
+
+    assert %Plug.Conn{resp_headers: resp_headers} =
+             :get
+             |> conn(route)
+             |> RequestCache.Support.Utils.ensure_default_opts()
+             |> put_private(:call_pid, pid)
+             |> put_resp_content_type("text/html")
+             |> Router.call([])
+
+    assert resp_headers === [
+             {"cache-control", "max-age=0, private, must-revalidate"},
+             {"content-type", "text/html; charset=utf-8"},
+             {"rc-cache-status", "HIT"}
+           ]
+  end
+
   test "allows you to use `cache` key inside opts to override specific cache for a request" do
   end
 end