Have our mirrors disable ETag

As spotted by Maone on https://mailman.boum.org/pipermail/tails-dev/2015-March/008346.html our mirrors all send very different “ETag” HTTP headers and that can break some download resume techniques like in Firefox (but not in wget for example).

Could we ask all our mirrors to disable that?

Original created by @sajolida on 9022 (Redmine)

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