changes to provide testing of HTTP headers modifications

Hi Landro,
First, thank you for introducing package TesTcl. I was looking for something to support iRules development...
To support your project I'm sharing several changes I made for my needs.

I found simple mocking of "HTTP::header insert" not enough for testing my iRules, so I added original HTTP::header proc simulation.
There is not always need to define end state if we have to test scenario when there is no action taken in specific condition.
My proposal is to make endstate oprtional and to provide additional verify command to test specific conditions after iRule execution. In my case it was needed for iRule dealing only with modification of headers.
I hope it will be clear when you look into example_irule_headers.tcl

Best regards,
Seba.