html5lib can set a multivalued attribute twice, and the tree builder can't handle that

[Leonard Richardson]

As seen here:

https://groups.google.com/forum/#!msg/beautifulsoup/lcgw7rq152A/x1QhmKaxcGkJ

When rearranging the parse tree, html5lib calls setAttributes multiple times. The first call split the value of the 'class' attribute into a list, and the second call passes in that list. The tree builder needs to be able to handle an incoming attribute value that's already a list.

[Leonard Richardson (leonardr)]

Fixed in revision 239.