Give possibility to override all metadata fields (#129)

Including `app_id`, `content_type` and `timestamp`. At the moment these values can not be modified by the user.

**Implementation details:**
Just reverse metadata merging, so custom values can override default ones.

lib/publisher.ex

@@ -334,10 +334,10 @@ defmodule GenRMQ.Publisher do
   # Put standard metadata fields to top level, everything else into headers
   defp merge_metadata(base, custom) do
     {metadata, headers} = custom |> Keyword.split(@metadata_fields)
-    # take "standard" fields and put them into metadata top-level
-    metadata
-    # put default values, override custom values on conflict
-    |> Keyword.merge(base)
+    # take "base" fields and put them into metadata top-level
+    base
+    # put custom values, override base values on conflict
+    |> Keyword.merge(metadata)
     # put all custom fields in the headers
     |> Keyword.merge(headers: headers)
   end

test/gen_rmq_publisher_test.exs

@@ -102,6 +102,19 @@ defmodule GenRMQ.PublisherTest do
       assert [{"header1", :longstr, "value"}] == meta[:headers]
     end
 
+    test "should override default metadata fields", %{publisher: publisher_pid} = context do
+      message = %{"msg" => "msg"}
+      sent_metadata = [app_id: "custom_id", content_type: "text/plain", timestamp: 1]
+      :ok = GenRMQ.Publisher.publish(publisher_pid, Jason.encode!(message), "some.routing.key", sent_metadata)
+
+      Assert.repeatedly(fn -> assert out_queue_count(context) >= 1 end)
+      {:ok, _received_message, received_metadata} = get_message_from_queue(context)
+
+      assert received_metadata[:app_id] == sent_metadata[:app_id]
+      assert received_metadata[:content_type] == sent_metadata[:content_type]
+      assert received_metadata[:timestamp] == sent_metadata[:timestamp]
+    end
+
     test "should publish a message with priority", %{publisher: publisher_pid} = context do
       message = %{"msg" => "with prio"}