change assign_mapper to not monkey patch user defined methods

[Former user]

If a user defines a method called 'update' on a class, then uses assign_mapper on that class, the 'update' method will be overridden, which may or may not be expected.

-Simon Wittber

[Former user]

patch for assignmapper.py

[Mike Bayer]

thanks, also sorry this took so long..implemented for the 0.4 branch in changeset:2994

[Mike Bayer]

Removing milestone: 0.4.0 (automated comment)