Quickfix, replaces thumbnail_url with polymorphic_url

[wiesion]

As described in rails/rails#15840 - *_path methods should not be allowed within Mailer actions.

The original code is in fact constructing an absolute URL, but is doing so by making using of a *_path method.
This is fine for web and data views, but not for Mailer views.

Rails' polymorphic_url exists since Version 2 and its syntax experienced no changes, so i assume it should be a backward compatible replacement.

[wiesion]

This should also address the issues #383 and #388.

[lauer]

thy not just use return thumbnail_url(thumb)
that works fine for me?

[alexbevi]

Closing this in favour of #406