toBoolean has inconsistent results for functions with 0 arguments and for functions with more than 0 arguments

[capaj]

I have a plnkr demonstrating the issue here:

http://plnkr.co/edit/MeivE2RRsRFKF3n05vwX?p=preview

To boolean should return consistent results for any function, no matter how many arguments it has.

[btford]

I like the changes suggested by @lgalfaso, but I think they only make sense in a 1.3.x release. To immediately fix the issue for 1.2.x, I think changing the behavior of toBoolean to explicitly make all fns truthy is the right thing to do.

I'm working on a fix here: https://github.com/btford/angular.js/tree/gh-5414