`If-Match`, `If-None-Match` and `Vary` headers are either a "*" value meaning that the header
matches every possible item or a list of items, one of them must be matched to fulfil the condition.
BREAKING CHANGE: `If-Match`, `If-None-Match` and `Vary` item variant name changed to `Items`
Adds strong and weak comparison to EntityTag as described in the RFC,
add tests for this. Make EntityTag safe to use by hiding the tag field,
this prevents users from inserting malicious values for the tag. Invalid
values can screw up header formatting and may allow to insert headers.
Introduce EntityTag::new(), .tag() and .set_tag() methods. Fix Display
trait for EntityTag. DQUOTES were missing. Remove custom formatting in ETag
header. Improve docs.
BREAKING_CHANGE: EntityTag.tag is private, use EntityTag.tag() and
EntityTag.set_tag("foobar") to access it.
