fixed options.xxx=false bug

Bugfix for accidentially overwriting sound constructor options that are set to false.

Currently if you create a new sound with 

````
var buzz_sound = new buzz.sound(sound_path, {
				formats: ['ogg', 'mp3'],
				loop: true,
				preload: false
			});
````

The sound file will be preloaded regardless. 
The extra test in the test suite will verify the bug. After pulling changes into src/buzz.js the bug is fixed.