Enable {agent: false} option override per request

Allow override of agent option to ```false``` for each request. 

Currently you can override an agent to true, or to an Agent object, but overriding to ```false``` will shortcut (via ```||```) to whatever the global setting is (```true```, ```false```, ```Agent```, or even ```undefined```).

In short, this pull request will make a per request ```false``` setting override the global setting.