Fix bug preventing product update

Product updates in the dashboard weren't being saved. This was a
regression introduced by my refactoring of the ProductUpdate/Create
views.

A test to ensure that a product title does indeed get updated has been
added.

Reported and fix supplied by @soloweb in #613. Thank you.
Merged @maikhoepfel refactoring
Merge branch 'issues/612/product_modify_fix'