sound.fade() doesn't trigger onfade if from parameter is string

[CorySanin]

If the from parameter is the string representation of a number, _volume is set to NaN and onfade never gets triggered. Strangely, the fade still occurs (which is why it took me several hours to figure out what was actually causing the problem). It should either attempt to parse the string as a number, or it shouldn't perform the fade if the parameter isn't valid.
Another thing to consider is that when the to parameter is set to a string, it doesn't seem cause any issues.

[goldfire]

Thanks, should be fixed now with 8362b75.