Always use UTC time

[Tratcher]

Ported from: http://katanaproject.codeplex.com/SourceControl/network/forks/uecasm/LastModified/contribution/8369#!/tab/changes

https://github.com/aspnet/StaticFiles/blob/dev/src/Microsoft.AspNet.StaticFiles/StaticFileContext.cs#L137

LastModified from the file info might be in local time. We should convert it to UTC before attempting any comparisons.

The switch to strongly typed headers at least fixed the header value output: https://github.com/aspnet/HttpAbstractions/blob/dev/src/Microsoft.Net.Http.Headers/HttpRuleParser.cs#L239

[uecasm]

The timezone offset of _lastModified is retained, so in theory it should do comparisons correctly regardless of whether they're local or universal. Though it wouldn't hurt to confirm that as I haven't used DateTimeOffset much myself.

[muratg]

@Tratcher Are we doing this in RC2?

[Tratcher]

Yes, we should do it.

[BrennanConroy]

e05469a