Adding logic to support function expressions for the onLoad listener

Also added tests for this feature. I ran the tests on recent railo - although I did test the little bit of logic separately on cf9 and 10.

This pull request does rely on version sniffing to ensure that isClosure() is available and supported (ACF 10+).  I did not see anything else in the code that used version sniffing in this manner - if there is a better way to achieve this let me know and I will resubmit.