Test for 'Origin by default', plus supporting tests & flags


* Add an '-insecureTLS' option, to disable cert checking
* Add a test to poll the CDN until it has successfully probed Origin 
* Add test code to check that a request will go to origin 'by default' (as in, not in the cache)

Discovered while writing another test that if we use `%q` for string values then `fmt` will add enclosing quotes. Which would make this read nicer when the header isn't set and `Header.Get()` returns an empty string.