Fix Pathname compatibility.

Until PR livinginthepast#42 was merged, String, as well as Pathname objects, were
supported just fine. Now, just String is supported, because Pathname
object does not support #start_with? method.

Therefore, this just naively converts everything to String to fix
Pathname support.

lib/fake_ftp/server.rb

@@ -154,7 +154,7 @@ def absolute=(value)
     private :options
 
     def abspath(filename)
-      return filename if filename.start_with?('/')
+      return filename if filename.to_s.start_with?('/')
       [@workdir.to_s, filename].join('/').gsub('//', '/')
     end