Checks for existence of String#underscore prior to overriding

Preventing overriding String#underscore if it already exists, e.g. ActiveSupport's.  There's a lot of functionality that depends on ActiveSupport's underscore implementation within any Rails application, namely ActiveAdmin.    Checking for the existence of the method prior to monkey patching would prevent overriding ActiveSupport's.  

I couldn't for the life of me get the test suite to pass locally prior to making any changes, so I'm not sure if it was already in a broken state or if my test account was incorrectly set-up.