Please review insensitive-lookup.patch:

* Adapt John’s patch to current code
* Apply similar changes to Request.remove_header()
* Expanded on case-insensitivity, previous capitalize() requirements, and header_items() casing in the documentation
* Dropped testing of unredirected_hdrs and of headers being case-sensitive. I think these are implementation details, and are not part of the implicit API from before the has_header() etc methods.
* Reverted unnecessary test removal in test case
* Added What’s New entry