Loosen header checks for case-insensitivity

I don't have any empirical evidence that sites may use different cases for their headers. I just figure, better safe than sorry. 

This also provide a way to easily get at the contents of the specified header with non-`?` versions of methods. So this adds tests for the new methods as well.
I fixed this in a new commit. It adds a test for secure cookies (using `github.com`) and also flushed out an existing bug with secure cookie detection.