net/http: support If-Match in ServeContent

net/http: support If-Match in ServeContent

- Added support for If-Match and If-Unmodified-Since.
- Precondition checks now more strictly follow RFC 7232 section 6, which
affects precedence when multiple condition headers are present.
- When serving a 304, Last-Modified header is now removed when no ETag is
present (as suggested by RFC 7232 section 4.1).
- If-None-Match supports multiple ETags.
- ETag comparison now correctly handles weak ETags.

Fixes #17572

Change-Id: I35039dea6811480ccf2889f8ed9c6a39ce34bfff
Reviewed-on: https://go-review.googlesource.com/32014
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>