This CL adds support to store arbitrary WebDAV properties in the
in-memory property system reference implementation. It covers the
the majority of property related litmus test cases. However, this CL
does not add support for COPY/MOVE/DELETE requests to the PropSystem
interface or implementation; memory occupied by dead properties of
affected resources will not be released. I propose to first agree on
how to store and lock dead properties in this CL and add support for
COPY/MOVE/DELETE in a follow-up change.

Before: Coverage of litmus 'props' test suite
16 tests were skipped, 14 tests run. 10 passed, 4 failed. 71.4%

After: Coverage of litmus 'props' test suite
0  tests were skipped, 30 tests run. 28 passed, 2 failed. 93.3%

Change-Id: Ie9af665fc588332ed30c7de256f47f8405078db3
Reviewed-on: https://go-review.googlesource.com/9374Reviewed-by: Nigel Tao <nigeltao@golang.org>