fix: send modification notification from AutoMap.Add() with existing key

[wlsnmrk]

Same basic bug and fix as #18, but sort of in reverse. Using AutoMap.Add() to update an existing key will now invoke callbacks registered via OnModify():

var myAutoMap = new AutoMap<int, int>();
myAutoMap.Add(0, 0);
myAutoMap.OnModify(() => Print("myAutoMap modified!"));
// will now print "myAutoMap modified!"
myAutoMap.Add(0, 1);

This behavior was failing previously.

Also added tests to verify that both OnUpdate() and OnModify() callbacks are invoked when an existing key-value pair is updated via Add().