This removes the need for someone to use the `time` crate to create a
date compatible with HTTP headers. It now works with the `SystemTime`
type from the standard library.
BREAKING CHANGE: `HttpDate` no longer has public fields. Convert between
`HttpDate` and `SystemTime` as needed.
The new macro handles single value headers, list headers, and list
headers with at least one item.
It creates the item for the header and contains its documentation. The
new macro allows handling
more header cases in the future, it will also be possible to include
tests inside the macro.
BREAKING CHANGE: Removed impl_header!() and impl_list_header!() macros,
use new header!() macro.
Using `time::Tm` directly in HTTP header fields requires special handling to parse and format
the header values., this stops us from using the header macros. By wrapping `time::Time` in a
`HttpDate`, we can use the `FromStr` and `Display` traits of `HttpDate` like for most other values.
BREAKING_CHANGE: All code using one of the `Date`, `Expires`, `If-Modified-Since`,
`If-Unmodified-Since`, `Last-Modified` header fields needs to wrap `time::Tm`
with `HttpDate`. Removed `FromStr` trait of `Date`, `If-Modified-Sice` and `If-Unmodified-Sice`,
implementing the trait here is inconsistent with other headers.
