previous attributes broken when another attributes changed in handler.

[mogadanez]

Scenario:

• has model with several listeners on some property propA
• one of the listener change another property propB.
• In rest of handlers for propA previousAttributes contains new value of propA instead old one.

In case of usage regular Model instead of RelationalModel previousAttributes contains old value as expected.

sample:
http://jsfiddle.net/4ohq1tho/1

this cause several issues like kmalakoff/knockback#121