IE 8-9 cache fix makes etag caching not working at all

[svlapin]

Hey guys,

first of all, thanks for the wonderful module!

Found that our app never GETs 304 for subsequent requests when it should to.
After some investigations I found that the reason is https://github.com/lynndylanhurley/ng-token-auth/blob/master/src/ng-token-auth.coffee#L920 which sets If-Modified-Since header to Mon, 26 Jul 1997 05:00:00 GMT $http-widely. This makes browser not to set If-none-match header even if the previous response to the same resource had etag header provided (tested in Chrome 55, Firefox 48).

As we don't support IE8-9, I just fixed the issue by resetting

$httpProvider.defaults.headers.get = {};

From the more common point of view, don't you think it would be better if ng-token-auth set such If-Modified-Since header on per-request basis?
I'm not very familiar with coffeescript, but think I'll be able to come up with PR if you agree.

Cheers.