Fix ConversationsHelper#conversation_class with frozen strings

app/helpers/conversations_helper.rb

@@ -2,10 +2,9 @@
 
 module ConversationsHelper
   def conversation_class(conversation, unread_count, selected_conversation_id)
-    conv_class = unread_count > 0 ? "unread " : ""
-    if selected_conversation_id && conversation.id == selected_conversation_id
-      conv_class << "selected"
-    end
-    conv_class
+    conv_class = unread_count > 0 ? "unread" : ""
+    return conv_class unless selected_conversation_id && conversation.id == selected_conversation_id
+
+    "#{conv_class} selected"
   end
 end