Fix double form submit when using ENTER (fixes #72)

public/viewjs/quantityunitform.js

@@ -39,9 +39,10 @@ $('#quantityunit-form input').keydown(function(event)
 {
 	if (event.keyCode === 13) //Enter
 	{
+		event.preventDefault();
+		
 		if (document.getElementById('quantityunit-form').checkValidity() === false) //There is at least one validation error
 		{
-			event.preventDefault();
 			return false;
 		}
 		else