add test for immutability of CBL's returned property maps

See https://github.com/couchbase/couchbase-lite-java-core/issues/226.

Fails without
https://github.com/couchbase/couchbase-lite-java-core/pull/228.
add case testing client changes to CBL's cached rev properties

Currently failing, meaning that clients can give a map to CBL, then
change that map, modifying CBL's cached copy on future calls to
getProperties().

See https://github.com/couchbase/couchbase-lite-java-core/issues/226.
Merge pull request #336 from mjq/unsafe_maps

Add tests for CBL's cached properties getting changed by clients, currently failing.