fix: send modification notification when map indexer adds a new value.

[wlsnmrk]

Using the AutoMap indexer to write a new key-value pair into the map will now invoke callbacks registered via OnModify():

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

This behavior was failing previously.

Also added tests to verify that both OnAdd() and OnModify() callbacks are invoked when new key-value pairs are added via indexer.

(Thanks to @CaseyHofland for spotting this!)