Default to using current mapped class as owner if none found

Default to using current mapped class as owner if none found

Repaired regression caused in 1.2.3 and 1.1.16 regarding association proxy
objects, revising the approach to :ticket:`4185` when calculating the
"owning class" of an association proxy to default to choosing the current
class if the proxy object is not directly associated with a mapped class,
such as a mixin.

Change-Id: I87d0ac09f695dc285bd4bbe0a547f1d5ce23e068
Fixes: #4185