Combination of `If-None-Match` and `If-Modified-Since` does not work properly

[drsybren]

RFC 7232 states:

A recipient MUST ignore If-Modified-Since if the request contains an
If-None-Match header field; the condition in If-None-Match is
considered to be a more accurate replacement for the condition in
If-Modified-Since, and the two are only combined for the sake of
interoperating with older intermediaries that might not implement
If-None-Match.

However, the code in func IsFresh() does not take this into account, and happily returns a 304 Not Modified when the Last-Modified response header does not exceed the If-Modified-Since request header.

In practice this means that this scenario will fail:

• Download a file, store its metadata.
• The server restores a backup of the server's files, including the modification dates of the served files.
• Trying to download the file again will still return a "fresh" status, even when the etag is a mis-match.

I haven't run the unit tests, but this code seems to fix things for me:

	if etag != "" && ifNoneMatch != "" {
		return checkEtagNoneMatch(trimTags(strings.Split(ifNoneMatch, ",")), etag)
	}

	if etag != "" && ifMatch != "" {
		return checkEtagMatch(trimTags(strings.Split(ifMatch, ",")), etag)
	}

	if lastModified != "" && ifModifiedSince != "" {
		return checkModifedMatch(lastModified, ifModifiedSince)
	}

	if lastModified != "" && ifUnmodifiedSince != "" {
		return checkUnmodifedMatch(lastModified, ifUnmodifiedSince)
	}

	return false