[FIX] Regression in image resize helper (Fixes #2529)

Commit 57ad514b makes the function preserve the aspect ration of the
original picture. Error of mine because the expected behavior was to
lose it for kanban view purpose.

For backward compatibility sake, this commit will keep the old behavior
by default.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32