Necessary as httpdate 1.0.3 introduced MSRV, choosing 1.56

Signed-off-by: Ayush Singh <[email protected]>
Co-authored-by: Sean McArthur <[email protected]>

* Add support for "must-understand" directive

* Address fmt issues

)

* Make authorization schemes case and whitespace insensitive

According to RFC7235[1]:

> It uses a case-
> insensitive token as a means to identify the authentication scheme,
> followed by additional information necessary for achieving
> authentication via that scheme.

[1]: https://datatracker.ietf.org/doc/html/rfc7235#section-2.1

* Use eq_ignore_ascii_case() for Authorization schemes

This is effectively the same as comparing the result of
`to_ascii_lowercase()`, with the benefit of avoiding "allocating and
copying temporaries" (according to the Rust `std` docs[1]).

[1]: https://doc.rust-lang.org/std/primitive.slice.html#method.eq_ignore_ascii_case

---------

Co-authored-by: Simon Bihel <[email protected]>

An HTTP Range of `bytes=-100` means a suffix, the last 100 bytes. This
was wrongly parsed as the Rust range `..100`, which means the first 100
bytes.

This has been fixed, but doing so required change `Range::iter` to
accept a length argument, to determine if the ranges are satisfiable.

BREAKING CHANGE: Change `.iter()` calls to `.satisfiable_ranges(len)`.
  Also, the `Range::bytes()` constructor will now return an error if
  pass a `RangeTo` (e.g. `Range::bytes(..100)`).