Make .attr() keep properties defined as serialize:false

[marshallswain]

I've started a test for this here: https://github.com/canjs/can-23/pull/10/files. Will be fixed by #10

[marshallswain]

When calling .attr() with no arguments, it internally calls mapHelpers.serialize with the how prop set to 'attr', here:

The serialize method calls map.forEach, here:

Inside forEach, the getOwnEnumerableKeys method is called:

This eventually runs the version of can.getOwnEnumerableKeys from can-map-define.

On line 444, above, calling keysForDefinition removes any attribute defined with serialize:false:

[marshallswain]

@justinbmeyer Maybe we can make a canReflect symbol for keysForDefinition to replace the one in the last screenshot?