grpc-js-core: use Buffer.from in metadata cloning

[cjihrig]

When cloning binary metadata, use Buffer.from() instead of Buffer.prototype.slice(), as the latter creates a new Buffer that shares the same underlying bytes.

If a true clone is not needed, then the same Buffer instance should be returned, and I can update the test. Note that the existing test suite (minus the test added in this PR) passes with v.slice(), v, and Buffer.from(v).