Support for ETag and If-None-Match headers

[thib92]

Hello,

I love your library, it's a really nice plug-and-play addon.

I could not find anything in the docs regarding ETag and If-None-Match HTTP headers. I am using those to diminish network traffic between my app and my back-end.

Are these supported out of the box? If not, I'd be glad to try and implement this feature.

[abbasghomi]

Please take look at this PR #32