StaticFileContext.LastModified should be a DateTimeOffset

[mattjohnsonpint]

To prevent time zone ambiguity, LastModified on StaticFileContext should probably be a DateTimeOffset rather than a DateTime. Otherwise, there's no clean way to ensure that the value was obtained through a UTC-based source.

This first needs to be fixed in IFile and IFileSystem (and implementations) as discussed in aspnet/FileSystem#9

This is especially important since the RFC 1123 ("r") format specifier is being used - as it may be converted with the wrong timestamp. (See notes on the other issue)

[Praburaj]

Dup of aspnet/FileSystem#9?

[Tratcher]

35ec5be

[mattjohnsonpint]

👍