This change adds ETag headers to GET/HEAD/POST and PUT responses. It does
not update the existing If-header request handling. The ETag header and
DAV property value can be overriden by implementing a custom property
system. A similar scheme is provided for Content-Type.

This CL makes net/webdav pass three more litmus ‘locks’ test cases
successfully.

Before: Summary for `locks': of 30 tests run: 27 passed, 3 failed. 90.0%
After:  Summary for `locks': of 34 tests run: 30 passed, 4 failed. 88.2%

Change-Id: I5102b9ac18d20844deaaa630b62cc3611b3f0740
Reviewed-on: https://go-review.googlesource.com/4903Reviewed-by: Nigel Tao <nigeltao@golang.org>