EntityTag type is private

[jdm]

For implementations like Servo that are implementing an HTTP cache, it would be nice to reuse the ETag machinery that already exists in this repository. Right now it's impossible to do anything with the typed ETag header and we need to reimplement the weak/strong comparisons.

[seanmonstar]

Yea, I'd be happy to add to the API! Is there need for the internal EntityTag, or can we just make the related headers accept an &Etag?

[jdm]

I don't think there's any particular need for the internal type.

[jakelee8]

EntityTag is still private for v0.3.8: https://github.com/hyperium/headers/blob/master/src/util/entity.rs