Loosen header checks for case-insensitivity.

Also provide a way to easily get at the contents of the specified
header. This adds tests for the new methods.
Merge branch 'master' into loosen-headers
add failing test for secure cookies
fix a bug I introduced, but also another bug
be DRY by referencing own methods
Merge pull request #10 from benbalter/loosen-headers

Loosen header checks for case-insensitivity