Allow sorting by multiple columns

[elad]

Hi,

Consider a table with first name, last name, and some boolean field. I'd like to be able to sort by the boolean field (true first, false last) and then sort by first name and last name.

That should be possible with ['toggle', 'first_name', 'last_name'] but the code has assumptions about the predicate being a simple string.

What's the reasoning? Any chance you're willing to fix it? :)